I don't think Quinn is that bad, she has a lot of bugs from the very few times I've played her but she's overall a great ADC or good against matchups with AD champions since her Q + E will allow them to pretty much retaliate with nothing and her burst isn't a joke, and a fed Quinn is ridiculous because all she has to do is deactivate her ult and auto attack you once or twice for a kill.
She needs a few small buffs and for her bugs to be fixed and I feel like she'd be a perfect ADC. She's pretty safe with her W vision and her Ult movement speed and even her E can make sick plays, great burst, scales pretty well into all phases of the game and can work with a lot of builds. She has clear counters and poor matchups, but also matchups she does amazingly in.