There's actually a case for Riot not to sexualize base models of champions too much.
Money.
Imagine being a horny 15 year old with your mothers credit card. You start playing this free game and there's this hotty Miss Fortune that looks really cool and is easy to play.
15 year old loves MFs boobies but OH MY GOSH!! Look at that skin!!!! 975 RP and you get a sexy MF cowgirl skin that shows half her stomach. TIme to use mommies credit card and buy sexy girl.
Riot profits and we can make the female characters more bad-ass and less ass (I tried making a joke I'm sorry)
could all be categorized as "Overly-attractive", but their not because people like you only see boobs and ass. You don't see girls making posts complaining about Kayn's pecs.