Not sure why you think "supergirl" is any more stupid than "Superman". Less original, sure, but nothing would indicate that a Superman movie would be inherently better/less stupid than a supergirl movie. The supergirl movie also made a point of showing how distinct it was from the Superman movie by not even taking place on the same planet. There was even a clip of her in the Superman movie showing her in direct contrast to him.

It's fine to say the movie sucks, but if an audience is turned away by just her name, I do think there's an element of misogyny there. Makes me wonder if "superboy" would do better instead. The character was imagined at a time when things centered men and so her name is defined by that, but her actions are not just Superman redux. She has her own story, but it's possible her name keeps her in his shadow for general audiences.