"Since this Pokémon is weakened by sunlight, it is rumored that the veil is used for protection. It is believed that seeing its true form will cause a mysterious illness. Mimikyu's disguise is modeled after Pikachu because of the popularity of Pikachu-styled merchandise. This lonely Pokémon thought that this disguise would allow it to make friends with people."
https://bulbapedia.bulbagarden.net/wiki/Mimikyu_(Pok%C3%A9mon)
https://bulbapedia.bulbagarden.net/wiki/Mimikyu_(Pok%C3%A9mon)