The reason for the Mirrorcle World butterflies :O

[tokyoxjapanxfan]

that's an interesting take on the butterflies

here are my 2 cents:
butterflies can also be a sign of rebirth/transformation--they go from an ugly little caterpiller to a coccoon to a butterfly. so maybe ayu is naked as a way to show she's "transformed" from her old self, to her current self?