You can actually see how many times a creation has been downloaded from the gallery, it works the same on both the website and the ingame gallery. 🙂
It's the number by the folder icon under a creation:
As for some sort of reward for creation popularity, I agree it could be a great idea to reward content creation, but I see a lot of potential problems and ways of trying to abuse the system, which would probably require significant dedicated resources to implement and maintain.
Nonetheless, it's an interesting idea and I suggest you share it in the Suggestion forums over on the official forums, which is dedicated exactly for these kind of ideas. 🙂
