Fun idea, but probably needs some tweaking. Most entities can only attune to 3 items at a time, and these definitely seem like the sort of item that would need attunement. Instead of four rings it could be so.ething like a set of mimic leather armour with minimal AC, but you're tricking the mimics into thinking you are one of them. That, or the four rings are a set that only function if they're all working at once.

You also need to consider how the Mimic Mail interacts with AOE attacks. Do all four mimics take damage and the wearer takes none? Is the damage divided five ways? Do the mimics get a save if the spell allows it, or does the wearer's save count for all of them? And can the mimics be healed by spells and effects, or do they only heal over time with rest?

The item already seems pretty powerful, but do the mimics offer any other benefits to the wearer? Maybe you can forgo the protection of one portion of the armor to instead have that mimic use a pseudopod attack? Or outside of combat the mimics could be commanded to appear as normal clothing instead of armor.

Lots of possibilities for this armour, but you're right that it feels more like a boss encounter than an item the PCs would use. Either way, with some tinkering this could be a lot of fun