It's interesting that the most complex and human love interest in the game is so divisive. Quinn comes from the most broken home with the least wealth, no family upon whom she can depend, no real friends, and a survival instinct she learned from her surroundings. Growing up in an environment where people have to have their fix at any cost or sell to people who have to have their fix at any cost teaches a person that being like the Jill's or even the Sage's of the world gets you hurt.
Quinn doesn't even know what real love is because she's never experienced it. Her entire life has been about what people can get from her and never about what anyone can do for her without a quid pro quo expectation of something in return. The MC could be her first true love interest in terms of someone who is not using her for anything and who genuinely likes her for the good qualities he knows she has behind the offensive sword and defensive shields she creates to keep people at a distance.
People don't come without baggage. Just because Quinn's baggage is more ostensible doesn't mean she is more flawed. Quinn's story has been told to a greater extent than has the story of other love interests. Perhaps when their stories are told they will be equally flawed in other ways. It might be best to reserve judgment on characters until the story has been told in its entirety.
I wouldn't say Quinn is the most complex character, she's a pretty standard tsundere, a stock character found in most Visual Novels and Dating Sims.
For the Halloween party, DPC should have Quinn go as Rin from "Fate Stay/Night" or Asuka from "Evangelion" to really hang a lampshade on her stock tsundere status.