I have indeed noticed what you're referring to. I guess my point is, she may be showing a bit more skin a bit more often nowadays, but I don't see it as being truly suggestive. Sure, Nintendo could've probably covered her up a bit more in games like Mario Strikers, but I think what she's wearing, most of the time, is perfectly appropriate anyway. Girls tend to go for the bare midriff and short shorts sometimes (not all, but some). Maybe that's just Peach's casual style. The creators could've dressed her down, but I don't think it's a big enough problem for that to be necessary.
Regardless, in Brawl, this is hardly the case. She's as classy as can be, and if she shows a bit of thing when she twirls around for a fighter stance, then so be it. Girls wear thongs. It happens. I, personally, would be much more disturbed by the lack of body parts than a little bit of cloth-covered hip.
But, I guess that just comes down to taste. Short of putting Peach in a bikini 3 sizes too small for her, I think she's fine.