Really, we wouldn't mind if Maniac Magee were told in the first person. There are definitely times we'd like to know a little more about just what's going on in that wacky head of his. But, it's just not to be. We've got a third person narrator here, so we see Maniac's journey unfold from the outside. Our only glimpses into what he's thinking come from his actions, like when "he got some paint and angrily brushed over the 101" on the door to the baseball equipment room after Grayson died (33.3).
It's probably better this way. Maniac is a legend, right? And you can't be all up close and personal with a legend. The third-person narrator lets us experience Maniac just the way all the other kids do: through stories.