Character-Dependent
Since the narration pulls a switcheroo between each chapter, the style changes pretty dramatically throughout the book depending on who's "speaking." For example, when Lipsha Morrissey is speaking, the narration is less formal and (it seems) more typical of Lipsha's speaking style and perhaps education—put more bluntly, he doesn't talk fancy.
Check out when he's talking about his grandmother, who was pressuring him to use his "touch" to help her out with Nector: "…when she mentions them love medicines, I feel my back prickle at the danger. These love medicines is something of an old Chippewa specialty" (13.1.53).
As you can see, Lipsha isn't the resident grammarian, and his narration reflects that fact. Also, his narrative gives you a sense of how emotional he is, compared to the other characters; you can kind of see that even in that little snippet of text about his "back prickle."
But then check out what happens when we mosey on over to Albertine's mind:
"Patient Abuse." There were two ways you could think of that title. One was obvious to a nursing student, and the other was obvious to a Kashpaw. Between my mother and myself the abuse was slow and tedious, requiring long periods of dormancy, living in the blood like hepatitis. When it broke out it was almost a relief. (1.2.2)
This quotes is miles away, style-wise, from Lipsha's voice. We hear her level of medical education ("living in the blood like hepatitis"), her studious nature ("There were two ways you could think about that title" sounds like a quote from a university essay) and her reserved nature ("almost a relief" doesn't sound like something a hyper-emotional person would say).