[MMM]

But a costume is when you are dressing up to try and be someone else. A tuxedo can be a costume if I am trying to be, say, James Bond.

I don't think the clothes themselves define whether an outfit is a costume or not, but the intentions of the wearer.

Lady Gaga's outrageous outfits are not generally called "costumes" because they help define her own personality.

Therefore if someone dresses in a lolita style, if she is dressing that way as an expression of her personality, I don't think it is fair to call it a "costume".