Do you think Hazuki being told to her face "yeah, your mom is fucking the girl you like, but your whole life is ahead of you, and your mom's just really very sad and lonely so just get over it" would just be fine?
Because fuck Hazuki and her feelings, right? Her own mom trampling on her this entire series is nothing, of course, because it's all about Mizuki after all. One-sided or not, Hazuki doesn't know that, and Mizuki even knows about her own daughter's feelings. It's categorically a fucked-up thing Mizuki is doing to her daughter, which by definition removes her from the "great mother" category.
This is going to have a "happy" ending, because realism isn't factoring into how any of this goes, and I'd bet money that Hazuki is just going to magically forgive her mother for doing all of this once she finds out--if she's even allowed to, which I also kind of doubt because it'd be very hard to write a realistic reaction for her that doesn't see her running away from home.