Mantra has described what "get away with" means. It does very commonly refer to something negative, but it can also mean something neutral.
So, "Art is what you can get away with" means that as long as at least one person is prepared to admit that what you've made is art, then you've "gotten away with" making art. It's a bit of a slightly humorous and cynical and witty thing to say. You could say the same thing of singing or poetry or dancing. A beautiful painting from centuries ago by one of the Old Masters is going to be considered art by everybody. There's no controversy there. But Tracy Emin once displayed a piece of art that consisted of a bed that she'd slept in and some stuff scattered on the floor next to it. If everybody who saw that had said "that's not art" then she wouldn't have made art. But because at least one person accepted it as art, she "got away with it".