I like this book because it is about the plight of gifted kids, among other important issues. Oskar is gifted, and no one understands him. It's pretty sad when a kid's own mother sees him as kind of a freak, or just plain does not "get" him. This is common with gifted kids.
I think the relationship between Oskar and his mother mirror the relationships of children today with their parents. Parents do not want to seem overbearing and by letting go a little the child, in this case Oskar, may look at the parent as not being an involved parent. Internally, like Oskar's mother, many parents are worried and keep a closer eye on their children than they allow them to believe.
They are definitely distant, but she is not uncaring. Even though he thinks she does not know where he is going on all of his quests for the "Blacks" in New York, she is quite aware and even calls ahead to let people know he's coming and to look after him. All the Blacks are caring, although a little odd, since they are all suffering after the 911 attack on the Twin Towers. In this way, his mother is actually mothering him, albeit secretly. She and Oskar are bonding in their suffering and loss of the husband/father in the family, but they do it in a different, less obvious way than most would expect.
Briefly, their relationship is lacking in that she is both preoccupied with her own feelings and lacking in empathy for Oskar; she doesn't understand his intellect or talents or personality. As a result, while he thinks of her welfare, they don't actually relate since she cannot understand what he needs for his welfare.