>>417466271
Some are, some aren't. Most aren't. The most popular avatars have well developed bodies, like Kikyo and Selestia. 
Some of the more loli avatars are like Anon, Karin, etc. 

I think that at least in English communities, the non-loli avatars are more popular. 
I know Maya is extremely popular on the JP side of things. She's weird because she has a weird loli potato head but her body is actually very similar to Kikyo's and is curvy.