The book is about M.C. Higgins, the Great. So, of course, the title of the book just has to be M.C. Higgins, the Great. Right?
After all, if your main guy is this arrogant, self-centered, teenaged boy, making the book title the same as what the boy actually calls himself is pretty appropriate. There's nothing that shows narcissism in a character as much as naming the book after a character who's way too into himself.
Which also, of course, sets us up for the whole fact that M.C. Higgins clearly isn't "the Great." He may be a legend in his own mind, but Lurhetta, the girl he crushes on, shows him just how not great he is, especially to his best friend, Ben, and Ben's family, the Killburns.
Getting taken down a peg, getting humbled—well, that's what the book is truly about. That way we can see how M.C. goes from the Great to just great. A guy who turns out to be amazing, but not because he's naturally so, but instead because he's willing to do the hard work of sticking up for his friend or building a wall with his hands.
Which is why, toward the end of the book, there's this sentence: "Good-by, M.C., the Great" (14.236). M.C.'s leaving his old, egocentric self behind.