Upon spawning or occasionally afterwards, a mime mob disguises itself as a node or entity that is close to it. The disguise is almost perfect for nodes (as entities are not shaded exactly like nodes … players can still figure out which sand or dirt node is really a mime) and perfect for non-animated entities. When copying animated entities though, mimes do not mimic the idle animations – they stand perfectly still instead. That sheep statue over there? Yep, that's a mime in disguise.
When attacking, a mime mob shoots globs of glue that can disorient and immobilize (and possibly drown?) a player. Once a player got out of the glue, a mime may have changed its disguise again. Fortunately, mimes are easy to kill if a player can dodge their glue attacks.