I have not read the first book in the series, so I came at this book with no prior knowledge of Joey or his relationships with family members.
Looking at the book from a Newbery perspective (the book received an Honor in 2001, and was a National Book Award Finalist), I can easily see that characterization was probably one of the number one attributes the committee was recognizing in giving the book an award. The main characters are all sharply and realistically drawn. Carter is trying to do what he thinks is right, he just is a failure. The grandmother is mean, but reading between the lines you can see that she really does care for Joey, even if he drives her insane sometimes. There are all too many adults in the world that just aren’t the nurturing type, and if she is not very nice, she’s also not malicious either. This is a dysfunctional family, but a realistic one.
Joey’s voice is very strong, and the sentence-level writing is clear and well done. There were some great analogies and good strong writing on display. Joey’s actions make a certain kind of sense when we view the world from his perspective, as each new idea builds on an old one, even at the point where a more rational person would stop and think “hmm, that’s maybe not such a great idea”.