"I couldn't aspire,
To anything higher,
Than, filled with desire,
To make you my own.
I wanna be loved by you, just you,
Nobody else but you.
I wanna be loved by you alone!"
-- https://www.youtube.com/watch?v=5eDHlgnRuaM

Mimikkyu getting inspired by a poster to become all that it can be. Its ghostly sheet with eyeholes cut-out seems to have scared too many trainers away and it feels severely dejected and misunderstood. It just wants to be loved by a nice Pokemon trainer, just like Ash loves Pikachu on the poster, so Mimikkyu will do what it must to garner your attention. Even if that means "refurbishing" this old dusty and torn Pikachu plush that's 20yrs old to remake it into a costume for itself. It tears off the hanging pieces to lovingly repaint the face by hand, it's not a good artist however, but it does its best.

Mimikkyu changed its appearance just so that people everywhere will love it like they love Pikachu, and with such a large Pikachu fanbase and Pikachu also being the official mascot of the Pokemon company, it's easy to see why. It doesn't want to be a scary ghost pokemon, it wants to be cute and to be cuddled. Heartbreaking, isn't it?