What is meant by "the kid who tattooed Giant John McNab"?

The quote in the question isn't quite right in the text's actual wording. The quote can be found at the beginning of chapter 8:

The kid who tattooed Giant John McNab's fastball for half a dozen home runs, then circled the sacks on a bunted frog.

The "kid" in the quote didn't tattoo John McNab. The kid tattooed the fastball that was thrown by McNab. The event that the quote is referencing occurs in chapter 7. Maniac is still new to the town, and nobody knows who he is. He wanders to the field where McNab is pitching and just dominating anybody who comes to the plate. Then Maniac Magee comes to the plate and plasters every pitch thrown to him to the outfield fence or over it. The kids can hardly believe it, and McNab is getting angrier by the pitch. McNab eventually decides to throw a frog instead of a baseball, and Maniac Magee bunts it for a home run. Chapter 8 begins by providing descriptions of this random kid who had just recently done all this amazing crazy stuff. The tattoo quote is one of the descriptions. The descriptions continue until somebody says that the kid must be some kind of "maniac." The name stuck, and that's how he came to be known.

Nobody knows who said it first, but somebody must have: "Kid's gotta be a maniac."
And somebody else must have said: "Yeah, reg'lar maniac."
And somebody else: "Yeah."
And that was it. Nobody (except Amanda Beale) had any other name for him, so pretty soon, when they wanted to talk about the new kid, that's what they called him: Maniac.
The legend had a name.