Every family living in poverty has a different story to tell and The Glass Castle, while it has some elements that are generally consistent with poverty in America, can be read as a somewhat unusual representation of poverty.
When we think about the poverty of a family, we think about two primary elements in America. food insecurity and housing insecurity. The children raid trashcans for food and more often than not, go hungry. Through the author's entire childhood, the family moves frequently, ahead of eviction, sometimes seeking refuge with relatives. So, in this way, the story depicts the elements that we know are part of poverty in America.
Depending upon how one sees Walls' parents, one may or may not see the family's poverty as created by a cause that clearly does create a great deal of poverty. If we see Walls' parents as mentally ill, and there is ample evidence of that in their actions, perhaps Rose Marie suffering from some sort of personality disorder and Rex clearly an alcoholic. People in America who suffer from mental illness, in a country that stigmatizes mental illness and fails to create an adequate safety net for it, can be too dysfunctional to hold down jobs or otherwise behave in ways that would keep them out of poverty. The Walls certainly have many elements of this sort of dysfunctionality.
On the other hand, if one sees Walls' parents as selfish, manipulative people, and there is a great deal of evidence to support that view of them, then the poverty that the family experiences is atypical of American poverty because most poverty in America is not self-inflicted. The Walls parents were educated people who could have held down jobs that were more than sufficient for the needs of the family and not been poor at all. One can interpret their characters as self-indulgent and their poverty as their own fault. This is seldom true of poverty in the United States. Most poor families are poor because of circumstances beyond their control, a lack of education, marginal jobs that do not pay a living wage, a lack of resources, no relatives in any better circumstances who could help, and probably, to some degree, a sense of despair.
One other way in which the Walls family differed from most poor families is that they did value education highly, and they provided the children with intellectual stimulation that the children in poor families seldom have. This is one of the saddest aspects of poverty, the impoverishment of vocabulary that children experience, the dearth of intellectual resources such as books and music, and the lack of parental role models for intellectual development, all of which leads to the perpetuation of poverty into the next generation.
All in all, while the Walls had the elements of food and housing insecurity in common with other poor families in the United States and possibly mental illness, their experience was in many ways atypical.