Duplica (Pokemon) is a girl that can disguise herself as anybody in matter of seconds.
Copycat, (Pokemon) like her name implies, copies people's looks just for the fun of it!
Roger (American Dad) loves disguises.
Sly Cooper is in the art of disguising and fooling others.
Sedusa (The Powerpuff Girls)
Changelings (My Little Pony Friendship is Magic)
Hydra (Marvel) has agents working all over the world. Masters of disguise, they can blend in almost anywhere as a group of assassins disguised themselves as police officers when plotting an attack on Nick Fury.
VFD members like Count Olaf (A Series of Unfortunate Events) are highly trained to become masters of disguise.
Robbie Rotten (Lazy Town) claims himself to be the master of disguise, having a costume for every possible scenerio