From the first line of the book, we hear humor in Junior's voice. He opens the story with a joke about him being born with water on the brain, and on the first page, calls himself a "weirdo." From these lines alone, we see Junior using a sarcastic and hyperbolic tone while he uses self-deprecating humor to tell his story.
The first few pages cover his physical growth and deformities from having hydrocephalus. He even admits his descriptions aren't "a very serious way to say it," but he finds the whole thing "weird and funny." He talks about having too many teeth, he talks about how big his head is, and he talks about his seizures, but he does so by using figurative language to make it sound funny and less serious.
We then learn about the meaning of the chapter title: The Black-Eye-of-the-Month Club. Junior admits he wants "to go outside," but "it's safer to stay inside" because he gets beaten up a lot. He says he stays inside and focuses on drawing, but what's important here is a tone shift. When Junior talks about drawing, we begin to feel a real connection. He isn't making fun of himself. He is genuinely talking about a coping mechanism that makes him happy and feel safe. Junior says he draws because he wants "to talk to the world." It's here, at the end of the chapter, that the reader gets to see through the humor and look into the heart of a kid that simply wants to feel connected to others.
Junior's sarcasm and self-deprecation make light of a very serious health issue, and we see this continue throughout the book. However, at the end of the chapter, we see the real Junior—a vulnerable kid who wants to make loving connections with people through his art. His tone shifts from self-deprecation to sincere and somewhat lonely.