Stargirl's reputation in the early parts of the book is not exactly glowing. It's not because she is mean or dumb or anything like that. She has a reputation for being the school's oddball. She stands out as different and unique. She is often bullied, but doesn't seem to notice or care. She is a bit awkward in social situations, too. For example, she cheers for the wrong team at school events and randomly sings happy birthday to people. She even puts on impromptu half time shows at football games.
Ralph Waldo Emerson would say this about Stargirl: "If a man does not keep pace with his companions, perhaps it is because he hears a different drummer. Let him step to the music which he hears, however measured or far away."
That's Stargirl. She marches to the beat of her own drum, and her peers at school do not understand it. So because of her outlandish clothing, behavior, comments, etc. Stargirl has the reputation of being something like the school weirdo. That does begin to change as the story progresses and people start to learn more about her talents.