Because Quinn is a character perpetually in the grey moral and legal area, she is also the most polarizing.
At a glance what with her being a pimp, drug dealer and possessing a mean streak she ticks all the boxes of your run of the mill antagonist. This is where many people write her off as the hellspawn sent to torture MC and MJ, enslave the HOTs and cast out Sage.
She is also easily one of the best written characters, and that depth in addition to her looks makes people at the other end really like her despite her flaws.
Her personality may not be to everyone's taste and that's perfectly fine. What I honestly don't see is how Quinn can be termed "a truly despicable piece of scum" as you put it, because while she has all the boxes which on the surface would indeed sway her image in that direction, if you really look closely at her actions, and the actions of other characters involved, she is not even objectively evil by a long shot given what we are privy to at this point in the story.
Her prostitution operation seems to be entirely consensual and voluntary, she does not coerce every HOT to join as they all seem to be ok doing it and even Mona backing out suggests she has that option in the first place. Time will tell how she handles that ofc, but it's not like she's sex trafficking her sisters - they can choose to sign up or not as far as we can tell. Same goes for Maya, it's normal for Quinn to be hazing her and so far she's totally oblivious to her restaurant meaning Quinn has not brought it up. Her omitting to dispel the free tuition rumor is not a flattering sign to be sure and will have consequences, however how she handles what happens next still remains to be seen.
Her smoking and selling weed is a victimless crime as far as I'm concerned, however being involved with shady individuals, dealing and using heroin or whatever that's supposed to be, that is what worries me. It's also probably going to be a major plot point in the story, and as such it holds a lot of potential for "redemption" as well as "damnation". It's not set in stone and could really go either way.
In essence, the stigma associated with being a drug-dealing pimp with attitude is worse than pretty much anything Quinn has actually done so far in the story. Imo it's too early to objectively call her an evil degenerate at this time because as far as we can tell she always has the willing participation of other characters in her activities. I don't believe she in fact is evil, but only DPC knows what he has in store for his characters. She could eventually turn out to be every bit as bad as some people say, even if it's only one among many versions of Quinn depending on MC's choices. But calling her evil scum at this point in time seems to be entirely a function of one's bias towards her grey area activities.
