This is an interesting angle on the whole argument about women's costumes, actually - the argument is generally about why the writers/authors are making them wear the costume, not the in-universe reason as to why the characters themselves would wear them. The idea that Black Canary dresses that way simply because it appeals to her sense of fashion is not a bad one - after all, if you think about it, a LOT of skin gets bared in women's fashion in general, from miniskirts to belly shirts. Yes, it could be argued that that itself is a result of sexism on the part of fashion designers, but it could also be a simple case of 'if you've got it, flaunt it' - and let's face it, Dinah IS a very beautiful woman. It's not too farfetched that she'd want to strut her stuff a little. (Of course, it was her Mom who actually came up with the costume, not her, but she kept the design, in any case.)