[Trexmaster]

I'm unsure that comparing spirit mirror to one of the worst accessories available to the player and nothing else is doing you any favors. That said, there are numerous other options that provide 'hidden' buffs of the same or higher magnitude, some even without Mysterious/Mask of Metal. For example;

Snake-Faced Helm immunity to item-based stun inflictions;

Crystal Mirror 50% light retaliation;

Fang-Faced Shield/Mirror Enchant reflect damage;

Heron Feather silence reduction;

Lucky Amulet's critical damage reduction;

Protective Paper Charm's status resist;

Ring of the Knight providing guard without a shield.

All of these can be deciding factors in a fight, but either come hidden or can be hidden in the same way; some of what I listed is arguably far more impactful, and 'by sheer merit of their existence' sway the fight if you're unaware of them. If you're REALLY having trouble with a spirit mirror user, I'd suggest packing a purge/requinite or some form of multi-hit projectile to deal with it. 

Spirit Mirror is fine in its current state. If you Kadouha yourself, that's on you.

TL;DR:

Spirit Mirror is not fine in its current state. It has no meaningful counter play beyond playing into it with a purposefully weak attack that not all setups will have access to, as a result conferring anyone who hides their status its benefits of forcing this counter against them whether or not the user actually wears a Spirit Mirror. Even if Spirit Mirror was 100% visible no matter what, people would still benefit from metagaming projectile-based builds as there currently are very few good plays against it, Splash being one of the few to gain some value (1m to apply weak, mirror breaking damage and a debuff that may help you). Nothing will change except people being able to fight against users with Mysterious or Mask of Metal without always having to worry that they have a pocket Spirit Mirror.

Circle ring is by no means good in general. Like Red Letter it serves a specific niche--hard countering a specific damage type at the cost of an appropriate detriment to deter people from always using these items haphazardly if it isn't something that they feel would be appropriate for their character. It's a fair comparison to Spirit Mirror in that it will grant a momentum swing in the user's favor if the matchup was appropriate to the item used (Spirit Mirror vs a projectile user, Circle Ring against anyone who uses Ice damage, Red Letter vs anyone who uses Fire damage). The difference is both Circle Ring and Red letter have counters of their own, both of them countered by Nihilist, which trades the users ability to absorb anything themselves to negate other absorbs, or even the counters built into the items themselves (Circle Ring's 9% increase damage taken, Red Letter's -75% Ice Resistance). Spirit Mirror does not, as Sawrock mentioned, have any meaningful counter beyond having a weak attack to trade with, sacrificing on average 3m. Its only weakness is expiring after one projectile--which considering how many things are 'one projectile' can be exceedingly powerful. Like you mentioned yourself Kadouha is just one projectile.

Nothing you listed aside from Protective Paper Charm is a factor in regards to Mysterious, though I'll go down the list one by one.

Snake-Faced Helm immunity to item-based stun inflictions;

Snake-Faced helmet's effect would've been relevant about 6 years ago when Skullcaver was completely unnerfed (24% chance to stun on crit) on top of Stun being entirely unnerfed (no DR and unable to escape it by resisting damage). Nowadays I doubt people even consider this an effect on the item with how rarely it will ever help you. I'm not even sure if it would stop Headcracker stunning you since it's technically not an item-based stun. If anything protecting against a successfully rolled stun is more beneficial to the attacker as they'll not need to deal with your 50% free DR.

Crystal Mirror 50% light retaliation;

Closest comparison I'd say if used against a Priest specifically. It's been brought up before when weighed against Spirit Mirror but the difference is that the opponent can potentially play around the retaliation damage by using other damage types assuming they didn't go all-in on nothing but Light damage. Spirit Mirror is a once and done thing and has far more potential to spike your opponent when it's revealed, whereas 50% retaliation will likely be worked around the moment its uncovered if the option is there. Not to mention the Mirrored enchant acts as a soft-counter to retaliation damage nowadays so most Priests will likely be using this knowing their high susceptibility to Crystal Mirror. Spirit Mirror does not offer any such thing. Yet.

Fang-Faced Shield/Mirror Enchant reflect damage;

Mirrored used to be a horribly overbearing enchant until it finally got made to require a 3m guard to activate. Considering the multitude of counters to Guard (guard break, dispel, knockdown) there are fair means of dealing with both of these. Fang-faced may still deal minor damage even without guard but even this can be overcome with Ghosthands if it's found to be overbearing to deal with. You will be actively aware Mirrored is up if your opponent uses Guard and can react appropriately if you have the means to do so. Fang-faced can catch you by surprise but its impact is often minimal and won't win you matchups you'd have lost otherwise without it now that multi-hits aren't as prevalent anymore/won't result in multiple triggers. If you spam 3m guards constantly that isn't Fang-faced winning you the fight, it's your opponents lack of a guard-break or knockdown.

Heron Feather silence reduction;

The obligatory CEL caster accessory. I won't deny this is super effective against Silence, as its intended to be, but knowing whether a Heron Feather is present or not hardly affects your decision to attempt to Silence a caster. You'll know whether it will be overbearing against you if you're going second against a caster, thus only 3 round or higher Silence effects will actually land on them. If you go second and only have 2-round silences odds are it's not worth trying. At worst you find out the hard way and cast a 2-round silence only to see it no longer active the next round, losing 3m in the process if that was the only thing your attack did. Whereas Spirit Mirror will cost you both that 3m and potentially a good deal of HP depending on what you threw into it.

Lucky Amulet's critical damage reduction;

10% DR vs critical damage. Certainly not bad for an accessory slot but not nearly as overbearing as Spirit Mirror. It's also a 9* accessory vs Spirit Mirror's 5*. Over the course of a fight you may mitigate roughly a single 3m attack's worth of damage from mitigating critical hits. Let's say you take 1,000 Critical damage over the course of a fight and still live to win it--you'd have taken in actuality 1,100 Critical damage, but only take 1,000 due to your accessory, saving you 100 damage. In contrast you'd have to have your Spirit Mirror activate against 50 damage or less to break even or worse compared to a Lucky Amulet for taking nearly all of your HP in critical hits.

Protective Paper Charm's status resist;

One of the only valid things when looking at what Mysterious can hide. Players who recognize high status res through something like Lust tattoo can potentially guess that someone's hiding a Paper Charm if their numbers are absurdly high (160-200+) on the initial check. Even if this blatant numerical display is removed it'd be a very common strategy to attempt to apply fire damage to remove the status effect--a counter to the Protective Paper Charm. Something Spirit Mirror doesn't have. You don't lose the damage you deal getting rid of a Paper Charm. On top of that, Status Resistance is extremely difficult to fit into most builds and Status Infliction often vastly exceeds it outside fringe cases like Glykin Priest/Summoners and Redtail Hexers. Protective Paper Charm definitely has a solid use for preventing average builds from consistently landing status effects but can easily be played around with something as simple as a Bomb thrown by a non-Engineer if the player wants to have a response to it. Spirit Mirror demands you spend a projectile on it or wait 10 rounds if you don't gamble on a dispel hitting it.

Ring of the Knight providing guard without a shield.

It will be immediately apparent once they cast Guard, and as stated above Guard has plenty of counters to work around it which provide more value for the attacker than the defender, unlike Spirit Mirror heavily punishing you for trying to disable it by any means aside from a very weak projectile or a dispel that actually manages to hit it.

With those out of the way I want to touch on a few other statements.

Some of what I listed is arguably far more impactful, and 'by sheer merit of their existence' sway the fight if you're unaware of them.

No, they really aren't, as stated above none of the listed accessories provide the momentum swing that a Spirit Mirror can without any recourse from those vulnerable to it to play around it. When I made that statement, I was referring to the fact that Spirit Mirror's existence alone confers its benefits to anyone who hides their status effects. Nothing aside from Protective Paper Charm can cause players to act differently against Mysterious because it exists, and Paper Charm is far more easily circumvented for those who may be impacted by it.

I'd suggest packing a purge/requinite or some form of multi-hit projectile to deal with it.

This is the problem. Why should everyone with a focus on projectile attacks be required to run up to anyone with Mysterious/Mask of Metal and immediately try to break a mirror that potentially doesn't even exist? At best you get a neutral trade against the Spirit Mirror tossing a Requinite at them and maybe hitting another buff off of them for a fair trade, at worst you deal minimal damage trying to use an ability meant to break mirrors with minimal losses. Not to mention in the case of relying on dispels, you could potentially remove every single buff someone with Mysterious has and not even be aware of it, thus still being in fear of a potential Reflect Projectile status if it doesn't blatantly show up in the list as dispelled. That's not even getting into how many things are curtailed by a Spirit Mirror which belong to classes that often don't have a good way to get past them or even a method at all outside throwing a rock and praying to their respective deities.

Imagine if a melee version of Spirit Mirror existed. Would you tell everyone who uses melee attacks to just throw Requinite at every single person with Mysterious and hope you actually purged Reflect Melee? Or to open every single fight with a charged Thousand Stab? (which honestly is pretty relevant even in this reality what with barrier tattoo existing).

I would not be against limiting Spirit Mirror's effectiveness VS players in exchange for functioning better against monsters, but all this thread was initially asking for was to make the status plainly visible, a change that would only impact those hiding their status effects and only in situations against projectile-users--who will still need to react appropriately to the mirror if used against them.

If Spirit Mirror won't be given a clear indicator, then we can have another alternative: have it break if the user takes non-projectile physical damage. So that there's actually counter play against it.