Children are pretty cruel, and being bullied for simply smelling weird/bad is pretty common, especially since at that age kids are willing to pick on you for no reason. I don't like Sooah, nor do I think her past justifies her current actions. However, being neglected and abused by your own parents, and then having the result of that neglect be the reason why you are bullied makes for a really shitty time. It's not really over the top, for me that Sooah didn't know how to shower; not learning basic personal hygiene because you were never taught is a thing common with kids who grew up in abusive/neglectful households. Actually, that, and general "dirtiness" or dirty appearances, is a common sign of child abuse that teachers, social workers, doctors, and other trained professionals look are taught to look for.
That being said, abused children who grow up usually learn how to pick up emotional/social cues more readily than others because they don't want to be abused anymore. People-pleasing is a big thing for those who've been abused (you can find a lot of literature on this), and it might be why Sooah - rather than bring unstable or cruel - manipulates. Being abused as a child leads to different behaviours as an adult, and not all adults who were formerly abused become villainous and terrorizing, harsh and violent.
I hope, though, that the author really doesn't show Mirae forgiving Sooah wholly and being and sparkles with her. Sooah was insidious and harmful, and I think Mirae deserves friendship with people much more than someone like Sooah who is still trying to justify shitty actions. Sure, more time on other characters would be very nice, as well as more development for Mirae and Kyungsuk, but I think in light of the time we also spent exploring Kyungsuk's mother's past, Sooah getting a handful of chapters isn't something so bad. For Mirae, at least, Sooah was/is still someone who affected her first few months at university a lot, and so that deserves some recognition as well.