There are many q&a sites out there, but don't know about any popular fashion q&a site. I can't understand what is the purpose for such a site as a general q&a site answeree or quora can deal with it as there is a fashion topic in these sites. If you are about to build such a q&a site then okay, but I can't see much scope in it. Moreover it is not a debatable subject or one that suits for a q&a theme. It's only my view I am sharing with.