My (otherwise great) roommate is taking a painting class and producing a lot of...errr... questionable art. It's not just about bad technique (which I would understand since she just started classes), it's more the subject matter that I object to. She likes to paint sort of violent or just weird stuff. I don't know if she thinks that putting images of guns and needles is edgy or artistic or what, but I don't want it hanging on my walls. How do I keep her from hanging up her (many) paintings?
Over the art
The good news is that it sounds like your roommate is still taking classes so maybe she'll improve her skills soon. In the meantime, I think you have to support her hobby — a little.
She has a new artistic endeavor and she should get to display her efforts in her own home. But since this is your home too, you could suggest she pick her best painting to "show off" in the living room and hang the rest in her own bedroom.
As far as subject matter, I'm guessing she is just experimenting with her style so I doubt the "edgy" thing will be a long-term problem. For now, be happy she's found something she likes and that this is your biggest roommate beef.