Of course I would prefer her in a starfleet uniform but for whatever reasons she isn't given one. What she is given is obviously acceptable ship wear since we see plenty of other Star Trek women dressed similarly. Kes wore skin tight stuff and no one complained. Why should it be fine for Kes and not fine for 7? Because 7 is hotter? Nope, doesn't fly.
Probably because it's more obvious. Kes was only in her catsuit a few episodes. And really, I think that was a poor move for her too and I'm glad that didn't become full time even by virtue of her leaving alone. Voyager was obviously trying to sex things up to get viewers one way or another.