I've noticed that the majority of pictures in showcases/ads that show the face of the provider do not feature a smile.
Obviously, there are exceptions, but for the most part, there's a "serious" look to most of them. Maybe this is meant to be sultry, sexy, but to me, it just comes across as dour in many cases.
With very few exceptions, I think any girl with a happy smile is prettier/sexier than that girl without.
Again, obviously, there are types of photos that do not lend themselves to a smile (BDSM, for example), but most, I think, could be improved with at least the corners of the mouth upturned, if not a full-on grin.
Maybe part of it is the self-shot photos. I would find it strange to smile at myself. (As opposed to laughing at myself, which comes relatively easily.) But it seems like a lot of the professional photo sets feature grim-looking girls.
Duck lips, on the other hand, well . . . Let's leave that for another thread.