Women dress up like that all the time here. I don't see the big deal, apart from the fact that it's really freaking nice to look at most of the time.
Tifa is a barmaid, it's natural that she'd want to show a little skin. Helps with tips and such. Doesn't make her a slut, much less a hooker.
Also, Tifa is nowhere near the most skimpily dressed FF female, nor is she anywhere near the first to be at least that skimpily dressed. Rosa, Rydia, Terra, Celes, Selphie, Beatrix, Garnet, Yuna, Rikku, Lulu, Fran, Ashe, and Penelo have all been routinely depicted wearing less clothing, or more revealing clothing, than Tifa.
Finally, it's as close to canonically established as possible that Cloud and Tifa have sex under the Highwind, but that doesn't make either of them "sluts," given that they've been in love with each other since their childhood and have only just come to realise each other's mutual affection. Given that the planet, for all they know, is just about to be destroyed, it's pretty natural that they'd want to consummate their relationship.