How does Mars Bar show that he dislikes Maniac?
Although you definitely notice from their first encounter early on that Mars Bar doesn't like Maniac, when he tries to yank the book from him and dares him to take a bite of his candy bar, the bully's dislike of Maniac becomes even more clear at the beginning of Chapter 16.
This is when Mars Bar actively shows his distaste for Maniac by trying to trip him as he runs in a game of baseball, by tackling him from behind even when they aren't even playing a game of football, and by riding his bike through a puddle right next to Maniac, trying to splash the dirty water onto him.
As the narrator explains, all these aggressive actions were Mars Bar trying to show Maniac how much he despises him. The thing is, Maniac doesn't get the message. It's not that he's stupid, it's just that he doesn't realize the meaning behind Mars Bar's behavior:
[Maniac] could see these things, but he couldn't see what they meant. He couldn't see that Mars Bar disliked him, maybe even hated him.
This is one example of Maniac's figurative blindness. He can't see (or rather, comprehend) that Mars Bar hates him, just as he can't see how his own white skin makes him an outsider in Amanda's neighborhood.