Quinn has been dominating with a 53+% winrate for over a year.
She currently has a 50+% winrate in all of her roles (Top, JG, ADC, wouldn't be surprised if she performs just fine in mid but there's not enough data), and has consistently been top 10, usually even top 5 for top lane. She's an incredibly safe pick, being able to deal with essentially every melee champion easily. The few close matchups where she doesn't just absolutely cuck her enemy with almost no chance of losing lane no matter how bad you fuck it up, she still has a higher win% because of her incredible roaming and turret taking. Most champions give her 1st blood turret before they're able to deal with her 1v1, and her early game is so insanely strong that she can keep most champions useless the entire game.
Quinn and Jayce have been the bane of top lane for a very long time, but Quinn has been dominating for even longer and I don't see as much talk about her. For someone with a fairly high play rate, she simply has too many favorable matchups. She's even capable of winning lane against mages and other ranged champions.
To have nearsight + a slow + an MS steroid + a knockback which puts her at max AA range from her enemy + 70-130% MS increase only requiring 2 seconds to channel from her ultimate is just too much for any top laner to deal with.
I honestly think the diminishing mana costs with rank on her ultimate should be gotten rid of so it always has a cost, and it should require longer to channel outside of combat. She's just too much of a lane bully to have her only weakness be ganks when she has several tools to evade 2v1s. Her kit consists of Q, E, and her passive, for a total of 100 mana. If she's going to be 100-0ing with one rotation, her mana costs should be far higher. She can afford to spam Q to harass, use E to disengage any unfavorable fight, and always have enough mana to pop her full kit.
tl;dr Give her a resource to manage and make her ultimate take longer to channel so there's actually a chance to force an unfavorable fight on her.