Quinn's bizarre obsession with Maya is a good example of why Quinn is the dumbest character in the game.
Quinn has been doubly fortunate so far in that Sage is too busy with her school work to focus on the HOTs, and the HOT code of "Don't turn on each other" is a perfect way for an opportunistic shitbag like Quinn to be shielded from criticism. But even when the stars align to give Quinn every opportunity to succeed, she still manages to ruin everything with her own incompetence.
Maya's slap is bound to raise a few questions that should by all reason make Sage investigate what's going on. Especially with Mona's abrupt departure as well. After all, Sage isn't a complete moron. When she suspects her boyfriend of cheating on her, Sage recruits the main character's help to learn the truth. If she does anything similar with Quinn, the truth would come out in a really big hurry. And in all honesty, it should've come out already.
Quinn is a bad character because other characters are forced to act out of character in order for Quinn's plot line to exist. The best example of this happens when you try to warn Maya about Quinn. There was no logical in-game reason for the MC to refuse to tell Maya why she shouldn't trust Quinn. She even flat out asked you why she shouldn't trust her, but you can't tell her the truth because that would destroy what is the flimsiest plot line in the game.
Quinn's delivery service is a pathetic house of cards that should've toppled ages ago. We could've ratted her out to Sage at any time, or at the very least threatened to do so unless Quinn eased off on Maya. But no, we aren't even allowed to try to do anything to Quinn. She's safely protected from the player by her status as the writer's pet. Maybe we'll get lucky and that will finally end now that the cat is out of the bag, but I'm not overly optimistic about it.