In Stargirl, who does Stargirl change for herself? Leo or Mica High?

changes for Leo. A good place to start looking for information supporting this is chapter 23.
chapter 23 sees Leo really starting to feel the repercussions of hanging out with Stargirl. He
is being shunned just about as much as Stargirl is being shunned, so he starts to avoid her too.
The two characters finally get together and talk in chapter 25. Leo keeps saying things like
"nobody" likes her, and he tries to blame everything on a hypothetical
"them":

"Because nobody likes you," I
said. "That's why. Nobody likes you."

"Nobody?" she said.
Her eyes covered me like the sky. "Nobody?"

I tried to play dumb,
but that wasn't working. "Hey," I said, "don't...