I think you're looking at it the wrong way. Quinn is a certain type of person, a practical, intelligent person who has very loose morals, probably because of how she grew up. She's also very brusque and abrasive, with a mean sense of humor. and she has a tougher time relating to people in an intimate way, not just sexually but emotionally, again probably because of how she's grown up.but the other LIs have pretty much ONLY sentimental problems, tomorrow all of them except Bella (maybe, her situation is not clear), they might as well marry MC and drop out of college and live happily ever after.
Quinn has other problems to deal with, and starts way behind the others.
I never said she has to become an angel but some little hint I would already expect to see, some less abrupt passage in the dialogue with Tommy for example, instead it's always the old Quinn.
on the other LIs the effect of MC's presence is obligatory, he will be their friend anyway, for Quinn it's a precise choice, he should have a greater impact, which I don't see.
then if BADIk will last 20 chapters there's plenty of time, but the impression is that Quinn's function is marked
I don't think she's necessarily going to change, her arc is going to be exploring those types of intimate emotions and getting a better handle on it with the MC, if I had to guess. She might become slightly less abrasive towards others but I don't believe looking at her like an onion that you peel layers off of to get to the inside is a good viewpoint, The way she is is just the way she is, and some guys like that type of person, girls too. They have many fun, and interesting qualities, they're just not nice. If you like that type of girl then Quinn is for you, especially if you pursue her, like I said earlier, she shows a very sensitive side that she's just not comfortable facing.
If you don't like that type of girl, which I completely understand, then she's just a villain for you, and that's fine, she works both ways I think.