Stargirl has a lot to say about love. There is the romantic love developing between Leo and Stargirl, sure, but there's also the love that Stargirl feels for humanity and the world around her. That love is an empathetic one. It's based on sharing in the feelings of others, and wanting to brighten their lives and lessen their pain. Plus, there's the fact that at the very end of the novel, Stargirl reclaims her identity through self-love. She realizes that you sacrifice yourself to make others happy. True love begins with self-love, to which we say, sing it, sister.
For much of the novel, Leo does not actually love Stargirl. He's infatuated with her, sure, and he likes her a whole lot. But love? No, he has to save that for when he grows up a bit more.
Stargirl knows that true love is all about acceptance. In her relationship with Leo, she couldn't really accept herself, because Leo couldn't accept her. But by the end of the book, she's totally cool with who she is. Leo is just too slow to catch up.