Well for starters, it's the name of the main character. But it's more than that. It's not like the book is called "Jeffrey Lionel Magee." It's also not called just "Maniac." It's not even "Jeffrey Lionel 'Maniac' Magee." So why "Maniac Magee?"
Besides the fact that everyone loves a little alliteration every now and again, Maniac Magee tells us that this is the story of a boy who has gone so far outside the bounds of a normal childhood that, in place of a first name, he's got a word that means "crazy person."
But Maniac isn't just some loony kid wandering the streets looking for another zoo animal to kiss. Nope, he's got a last name. He had a family at one point, and he's desperately looking for another, even if he doesn't always realize it. Maniac's last name comes from his parents, and tells us that this grimy wanderer belonged to someone once, and he can again.