Do all mimikyu choose to look like pikachu, or are there some that disguise themselves as other pokemon?

For whatever reason, mimikyu fixate on pikachu and pikachu alone (likely due to pikachu’s popularity as a mascot, actually, but that’s a subject for another ask). While it’s possible to give a mimikyu another costume and while it’s possible for a mimikyu to temporarily don another costume of a popular pokémon (such as clefairy) in order to attain that level of popularity, they’ll generally go back to their pikachu costumes rather quickly, I’m afraid.