Karen in Stargirl would really depend on how far the history of the JSA on Earth-2 got. Other heroes on that world did exist, the seven soldiers of victory, the all-star squadron, and the freedom fighters, so nothing specifically is saying that the cousin of a semi-retired Superman couldn’t be on that Earth either. Could be like the 70’s All-Star comics she just hasn’t made her official debut yet, maybe because of the death of the JSA.