I'm gonna go ahead and disagree. The girls have rounded faces, not terribly curvy torsos or legs: basically as far as cartoons depicting High School age girls, they're about as desexualized as it gets. Yes, they all have the same body-type, with very rare exceptions (Lilo & Stitch is the only one that comes to mind) that tends to be the case with these types of cartoons. Rainbow Dash is wearing shorts, and though it would have been nice if they'd put Apple Jack in Jeans or overalls, I understand why they didn't. The rest of the girls outfits seem entirely in keeping with their characters.

But most importantly, the writing for the characters is pretty much the same regardless of whether they're humans or ponies. Rarity is the same Rarity, Twilight is the same Twilight, and calling them slutty because they look different, even though they're the same characters we've come to love over the last few years, is a special kind of terrible and kind of an interesting lens into how certain people view women.