Well, if it's any consolation, the show runners stole Reactron (a Supergirl villain) and made him a Superman villain in the show.
But yeah, I get your point and I am sick of the showrunners constantly stealing from Superman. I looked up Supergirl's rogues gallery and there is PLENTY there to choose from. Aside from that, if Supergirl's background doesn't have anything to take then that is a GOLD MINE of creativity because, guess what? The show runners can make it up. But no, they show no originality. What Supergirl villains they actually DO use, they butcher up so they are completely unrecognizable simply because they are too freakin' LAZY to stick to the source material (The mockery that is Reign anyone?).
The show has been ruined for me as well. Let the ratings continue to drop and get cancelled already. I won't shed a tear.