Why is it fairy type?

Quotas.

Gamefreak need to shove the fairy type onto as many pokemon as possible, 1 generation and there's already more faggot types than ice types

Basic themes of deception and glamour that are common to both faeries and ghosts.

In terms of internal consistency, Mimikyu is trying very hard to be cute, which is what the Fairy type has been determined to be all about, so with its disguise it becomes Fairy.

>>33420830
Doppelgangers and changelings are technically fairies. If you see mimikyu as any of those, it's not hard to understand the fairy typing.

If introduced 2 gens ago it probably would have been ghost/normal which is far more interesting than ghost/fairy but I digress.

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>>33420830
Why does he wear the mask?

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>>33420985
>ghost/normal
that typing is reserved for Schrödinger's Meowth.