Reflective, Sympathetic; Desperate, Helpless, Frustrated
We know those aren't happy words, but don't despair. This novel might not be cheery, but it's not as much of a complete downer as you'd think. Keep in mind that the book is told from the perspective of someone looking back on the not-so-happy events of their life not because she is angry and vengeful but because she wants to understand why her mother might have done the things that she did.
That's what we mean when we say the tone is reflective (it looks back with new insight) and sympathetic (it tries to understand, not judge), especially when it comes to Bone's view of Anney.
Bone definitely has some strong feelings about other characters, but Narrator-Bone is all about allowing emotions and observations to speak for themselves. For instance, at one point, Bone wonders, "How long had it been since I had seen Mama not tired, not sad, not scared?" (14.14). Through this thought, we get a sense that Glen's aggression toward Bone is not unnoticed, or unfelt, by Anney.
At the same time, Bone goes through a whole lot of dark times, and it wouldn't be a good book if we didn't get a sense of that through the text. Here's just one example:
To say anything would mean trying to tell her everything, to describe those times when he held me tight to his belly and called me sweet names I did not want to hear. I remained silent, stubborn, resentful, and collected my bruises as if they were unavoidable. (8.38)
Talk about no way out. The tone here is trying to capture Bone's state of mind at the time—and trying to explain the reasons why she feels she can't tell Anney about the abuse. As readers, we too feel some of that frustration, as we witness a situation we can't do anything about. Yes, we want to curl up in a ball and not face the world. Yes, we want to pound our fists on something and ask "Why? Why?"—but that's the point.
Bone is in a situation where she can't reach out to the people around her, and as readers, we're in a situation where we can't reach through the text to help Bone. See what we mean when we say the tone here is "helpless" and "frustrated"?