He remembers all the fun times you had, back when the madness seemed harmless. Afterwards, the knowledge that he was the cause of it all drove him into hiding. But decades of loneliness weighed on him, until eventually he came up with a idea on how to have those fun days back.
The disguise isn't for his benefit, it's for yours.
Mimikyu is Missingno. That's why looking at what's underneath kills you: it corrupts your save file.
