When I describe The Glass Castle the first thing I say is that it is tragically funny. This is how the author comes across - though probably not intentionally. The thing about this story is that it is a tale of the author's childhood as she remembers it. The crazy things her parents said, did, and got away with were so common place to Jeanette and her siblings that as children they never considered an alternative.
She writes about them with the knowledge as an adult that much of what she dealt with as a child could very likely be classified as either abuse or neglect - but ironically, she never felt abused nor neglected as a child (in fact, she seemed content growing up in this environment of tough love). She knew her life was different from other kids' lives - but she also found her parents' differences to be as endearing and unique as they were strange and somewhat scary.
In this way, I might describe the intended tone to be straightforward and as honest as possible. She doesn't paint her parents in a light of particular respect or disrespect. Instead - she shows things the way they were, and allows her readers to come up with their own opinions. It was a bold approach to this memoir - one that the author feared would alienate her from a fan base. Instead, it probably made her more successful.