Uncle Dan and Aunt Dot don't get a lot of time in Maniac, but we feel their effect through the whole book. It's because of them that Maniac even starts out on his journey.
See, Uncle Dan and Aunt Dot were completely unable to provide a loving family to Maniac in the eight years that they cared for him:
Aunt Dot and Uncle Dan hated each other, but because they were strict Catholics, they wouldn't get a divorce. Around the time Jeffrey arrived, they stopped talking to each other. Then they stopped sharing. (1.7)
What does this say about who they are as people? It says that their hatred for each other is so strong that they can't see the effect their general ickiness has on the parentless child in their care. Only really bad people could be this self-centered and blind.
Maniac is constantly looking for a home. But how exactly does he know what a home is? And how has he figured out the difference between a house and a home? Well, for starters he knows that a home is a place where things are shared, not a place divided: "Two TVS. Two refrigerators. Two toasters. If it were possible, they would have had two Jeffreys. As it was, they split him up as best they could" (1.8).
From what we can tell, Dot and Dan took care of Maniac's material needs. He was fed, clothed, sent to school. But to end up screaming on a stage at age 11, he'd had to have known something was wrong. After all, it's not so unusual for a kid to split time between parents after a divorce. What's so different about Aunt Dot and Uncle Dan that Maniac thinks the only thing he can do is run away?