Why do you think Stargirl joined cheerleading?

This question is up to individual reader opinion. Readers are simply not told why Stargirl accepts Mallory's offer to join the cheerleading squad in chapter 5. What we do know by that point is that Stargirl is her own person. She either doesn't care what other people think, or she is completely oblivious to how people see her. She simply does what makes her happy. What we as readers will eventually come to realize is that Stargirl deeply desires to make other people happy. It is why she sends them random cards, sings "Happy Birthday" to basically everyone, and cheers for both teams. Cheerleading seems like a natural fit. It allows her to officially cheer for people.
You could also argue that Stargirl is completely aware of the fact that people think she is weird. She knows that cheerleaders are generally the popular girls, and joining the team lets her cheer without seeming odd.