[Chaos]

Well, Charm only comes from like three different sources as far as I know. Wink, Lunar Lunatism, and Sweet Leaf. It's not quite the as accessible as Ice Damage, Fire Damage, or even Fear or Hesitation, and we have immunities/absorptions for those effects. I think an accessory with just immunity: Charm would be fine. On the extreme end Charm is the most insane damage reduction in the game, so the item would be undoubtedly useful.

1. Fire and Ice Absorbs come with nasty drawbacks
2. Red Cape requires your HP to go down to a certain level, and even then, only gives you a temporary immunity
3. Braveheart demands that you give up your torso slot, and Hesitation has less sources than Charm, if memory serves me correctly
4. Shaitans are forced to stick with very low SAN in order to achieve Charm immunity, often leaving them with less HP/FP than others and very poor Status Resistance

No, it would not be fine. It's basically an unstoppable immunity to a potent status effect with no real drawback to the user, no requirements to fulfill, and no burden on the user's equipment slots. The most this item would be is Charm immunity on a Hands or Torso item. Any more than that, and you've got a free pass to not bothering with SAN/FAI when Charm pops up.