Riot intentions for Quinn?
I'm going to make a lot of comparison to Vayne here because I feel like they are meant to be similar in their ADC roles supposing Quinn is supposed to be an ADC at all. They both have a passive for extra damage, they both have a targetted knockback/slow, they both have strong chase mechanisms. Of course they do have some differences but these two kits I think are meant to function with similar purposes. Shorter range burst with good chase and mobility in fights. Yet Quinn is practically non existent in comparison to Vayne and when she is you're probably more likely to see her top than as an ADC.
I now find myself wondering did riot intend for Quinn to be an ADC? Does Riot still intend for her to be an ADC or are they now comfortable for her to be a top laner? If she's supposed to be an ADC that is competitive with Vayne her balance needs to be looked at. Maybe I'm wrong but I don't think anyone considers Quinn to be a strong pick as an ADC.
What can be done to make Quinn a competitive ADC? Well I can think of some obvious changes that might make her viable completely without any major changes. Change her E in human form so that she doesn't have to dash at her target, instead make Valor come in and slow her target with the ability to stun or disorient(fear mechanically) if marked while still leaving the target marked for auto damage, I know you already have Valor coming in with Q but something like this would increase her viability tremendously. Vayne for instance shoots out her knockback/stun/slow from range and it increases her survivability far greater than Quinn's. Technically Quinn has a tiny semi hard CC in the form of a 1 millimeter .1second knockback in human form but for some reason she loses this interrupt/slow in bird form where she actually needs a much stronger form of CC or other form of survivability since she has to get in melee range. The hard CC Vayne has really gives her a trump card over Quinn and without some form of comparable hard CC herself will never be able to compete. Quinn's passive is also weaker than silver bolts in the majority of situations especially when it matters for an ADC to have a steroid. Basically Quinn's passive is supposed to make her more of a lane bully than Vayne but in reality it doesn't work as well most of the time especially in bottom lane where her E puts her in a nice tasty CC range for the support.
I really think the major problem here is Quinns survivability, I don't feel that her chasing potential is that much stronger to justify the huge difference in survivability due to her constant need to get into melee range for her E and bird form, and to top it off her dueling potential is currently less than Vaynes especially after 6 where her bird form is supposed to make her a good duelist but instead is used to run, roam, or get back in lane most of the time unless the fight is already won and she needs the bird form to chase where Vayne has her constant passive. The roaming portion of it is useful but it's not enough especially when you need to be farming as an ADC. Heck even her mobility is less than Vayne's in a team fight regardless of bird form speed because it's not about how fast she can move it's about how useful her mobility is in a team fight. Vayn's tumble with stealth is far superior to Quinn's mobility and Vayne has a permanent built in mechanism for chasing and finishing off fights.
Basically something needs changed about Quinn's marking, Q, E, or ult to make her better in team fights. She is not on the level of other ADC's. In fact she's more of a squishy assassin than anything and I really think she needs to be looked at.
I don't hate Vayne, I think she has a kit that works well and that is what we all want, I just want Quinn brought up to her level as a competitive ADC. Again i'm comparing these two because I think they are supposed to be similar in role, I'm not claiming Vayne is a top ADC for competetive play, I am claiming however that Quinn is a horrible pick for an ADC in competetive play and probably wouldn't take too much work to bring her up to par.
Let's get some more ADC balance please, perhaps it wouldn't be so hard to find ADC's in team builder if we had a larger selection to choose from, that or get her role changed to fighter/assassin and give her more burst to support it.