You kjnow a while ago I found myself wondering how pegasi would steer, and now... I really don't need the mental image of a pegasi with a
rudder...
If we look for attraction from a biological standpoint, then qualities good for breeding and foal care would be attractive. Which would probably mean wider hips, maybe shorter legs... bit broader, and just generally sort of stout. Which, come to think of it, would make Mrs. Cake and Rarities mom, like, goddesses. Wow. There's a weird thought for you.
Still though, there's the cultural aspect of attraction too; what we're told is attractive (or, what they're told). We don't see much defined beauty in the show(which is a brilliant move for the kids - if we don't define what looks good, we're not getting any of that unrealistic standards for beauty crap[I know it's hardly relevant considering the shows cast, but still]), it's difficult to tell. But a lot of dresses and cloths show off the front legs, and those that cover the front legs are very form-fitting. So maybe thinner legs are pretty? Rarity also comments on not wanting to make Dash 'look like a tank', so we can extrapolate that rounder flanks probably aren't that attractive.
oh dear... this is sort of shaping up to look like thinner = better in Equestria, isn't it?