Quinn toplane main here.
So I’m super glad that you’re advocating for my main girl here. I would not say she beats every melee top laners hands down. While I know I still have plenty to learn about the game and about Quinn, I would say that she struggles against hard initiate and good base damage values. Stacking armor early game is also hard for her to deal with, since she’s auto-attack reliant and Ninja Tabi exist.
As an example, I normally do quite well against Malphite and Darius. However, I can recall in recent memory games where I got completely pubstomped by these two. The Malphite maxes Seismic Shard first and literally just kept chunking my health, and the Darius was very judicious in his use of Apprehend. Quinn’s definitely a good top laner, but she also struggles against hyper aggressive play, melee or ranged not withstanding.