There is no mystery behind the title of the novel. It is called Stargirl because its focus is on this one character and the impact she has on Leo (our narrator). The book pays homage to someone who means a great deal to the narrator, so what better title than her own, self-chosen name?
Ah, and that's just it. The title Stargirl emphasizes the fact that this is not a name she was born with. After all, what kind of parents would name their kid Stargirl? No, she was named Susan, but she renamed herself Stargirl because it felt… right. That should tell you right away that you are about to read a tale about a rather unusual young lady.
Calling the novel Stargirl also foreshadows one of the book's last scenes—the conversation between Archie and a grown-up Leo. In that conversation, Archie tells Leo that Stargirl is a star person, which explains her strange wisdom and deep connection to the world around her. For more on this, check out Stargirl's "Character" section.