On Rachel...her default dominatrix leathers? No. That's blatantly purely for fan service and doesn't make her look badass at all, especially since it looks both incredibly uncomfortable as well as highly impractical. Its part of why I always found her character so silly.
Her purple demon spikes thing could be argued as badass, in a golden axe/dragons crown warrior kinda way though, despite being more revealing.
Like just showing skin doesn't mean you can't be badass. I've always loved the Conan barbarian style, which factors into my love for midriffs. The catch is the outfit still needs to look appropriate and practical, not just sexy for the sake of sexy.
On Mila, its the difference between her awful pink string bikini, and the far cooler volleyball shirt with bikini bottoms. The former just looks stupid since its just sexy for no real rhyme or reason, while the latter is both sexy and far more appropriate for physical activity.
On the golden axe characters, its not so much a double standard so much as the fact that straight men find pretty girls attractive lol. They are both dressed barbarian style.
As for badass....ayane can pull most outfits off, with the notable exception of modern casual outfits. She can do skimpy armor, ninja outfits, etc etc, but she looks downright silly in that white beanie outfit from story mode, precisely because she's a highly dangerous assassin from a secluded ninja village, not a regular teenage girl.