It is interesting to note that the most interesting character, and the character for whom the novel is named, is not the narrator. Despite the title, this isn't really Stargirl's story; it's Leo Borlock's. Of course, it's a little bit Stargirl's story, too. After all, much of the book focuses on Leo's relationship with her, which shakes him to the core, forcing him to question all kinds of things about himself, high school, and life.
What's so great about having Leo as our narrator is that he is clearly an honest guy. He's not afraid to show us his flaws, and that means we can trust him as he guides us through the story. When he doesn't cheer for Stargirl in Chapter 30, for example, he freely admits that he was being "cowardly." A guy who tells us the truth about his own mistakes is probably telling the truth about everything else, right?
That means that Leo's descriptions of Stargirl are probably accurate, so no matter how outrageous she is, she's real. Leo tells us so. It also means that, because we can trust him, we can also relate to him. Who hasn't wanted desperately to fit in, and sacrificed a little bit of integrity to do so? Who hasn't fallen head-over-heels in love and lost sight of what's important? Leo is, in many ways, a typical teenager, but as we listen to him tell his story, we learn right alongside him that being typical isn't all it's cracked up to be.