Riven probably has the steepest mastery curve out of any champion in the game, and that will probably remain the case forever unless Riot wishes to address it.
In the general sense, Riven is probably a B or even C-tier top laner, but in the hands of someone who's dedicated 200+ games on her, I gather that is not the case. The issue is Riven's overall power is directly linked to how well the player piloting her is able to weave AA animation-cancels in her combos. The difference between someone who is able to pull it off and someone who can't will result in Riven either dealing not enough damage to trade evenly in lane, or straight-up one-shotting her opponent.
Obvious example, but people like Adrian (and a few others) are still able to carry themselves to Challenger using Riven, so clearly she is viable in the hands of people who can wield her correctly. The issue is that the skill curve to learn her is just way too steep. If Riot ever wants to buff her numbers to the point where she's meta she'll be completely busted in the hands of people who mastered her.