It might just be that you're seeing her less as you get higher, because she's not as easy to use as Ashe. I mean how can you argue with a stun that you don't even have to be in range of any enemy to use? People (myself included) who aren't great at carries, will play Ashe on occasion when the team needs it, because it fulfills the role, and provides utility even if you're stinking it up. Tristana on the other hand, doesn't provide as much when she's doing poorly.

any champion can be good in the right hands, although some are harder to use then others.