Think of it kinda like Sylveon where you need to play with Eevee on pokemon amie in order for Eevee to evolve into Sylveon. Same thing in this case, because Mimikkyu has grown to trust you and knows that you love them he takes off the Pikachu merch and you get to see how he really looks? But only because you actually put in the effort and loved them.

I mean, it’s just an idea I guess, but it would be a very heart touching thing for Pokemon to do, and I for one would like to see it happen.