She is Weaker than Superman
While Superman is basically invincible, that does not make him stronger than Wonder Woman. Whereas Superman is strong and sturdy — Wonder Woman is clever and agile. Furthermore, Superman legitimately has a weakness (Kryptonite), while Wonder Woman is truly invincible. She is, after all, basically a goddess.
And if that’s not convincing, then in the aforementioned plot line where Superman is controlled by Maxwell Lord — Wonder Woman literally beats him.
Wonder Woman was Always a Feminist Icon
While the creation of her character was meant to set an example for girls all around the world, Wonder Woman wasn’t always shown to be fierce, strong, and ready to jump into action. Sure, in 99% of her storylines she is completely in charge, there was that one comic run of Justice Society where Wonder Woman was, essentially, a secretary for the rest of the team. This happened in 1942.
Since then the DC team made sure something like this wouldn’t happen again. Good.
Wonder Woman is Boring
Wonder Woman doesn’t have the absolutely astounding popularity of Superman. Wonder Woman also doesn’t have the brilliant roster of villains of Batman. Wonder Woman really cares about diplomacy and peace in the world. Somehow, these three factors lead many people to believe that Wonder Woman is boring.
But this couldn’t be further from the truth. Wonder Woman comics usually have great stories. Wonder Woman is witty, confident, and also very funny in her own way. The juxtaposition of her Amazon past and her introduction to the human world was shown briefly in the movie, but it is explored in much more depth in the comic series. And, in this author’s humble opinion, Wonder Woman’s path towards understanding humans is much more interesting than Superman’s.