The fashion police.
Fashionista is the lexical combination (see 'portmanteau') of the words Fashion, which is the socially decided (see 'groupthink') garb to be worn by those wishing to be accepted in high society, and Sandinista, a Nicaraguan authoritarian political party.
Despite recent trends in attempt to make this term more acceptable it must be noted that its etymology lends major negative conotation and it is ill-advised to use this word as a positive qualifier.
"Did you see what she was wearing? I'm no fashionista but those are the ugliest shoes I've ever seen."