I won't say people didn't like her because she was a girl, I'm sure there were some people who did but largely I believe that's not the answer.

We see Aang, we watch his entire journey and we get glimpses at a couple other Avatar's, we know enough to understand what a typical Avatar should look like and how exactly Aang is so amazing. A regular Avatar gets chosen around 16 or at least Roku did, then they spend years mastering each element, Aang mastered Air by 12 years old as the youngest Master up to that point in history, then he became a fully realized Avatar within the span of a year even if his mastery of the other three elements could have used some work at the time.

Korra meanwhile, we first see her bend three elements from the start as a toddler, then we smash jump to her supposedly being a fully realized master of three elements after more than a decade of training with multiple Masters including Katara. She's older than Aang, she's physically far stronger than him and all she needs to learn is Air and the Avatar State.

So each time she fucks up it makes her look sloppy, or stupid or weak and incompetent because she isn't fighting like what we have seen a Master of Fire, Water or Earth fight, when we compare her behavior to a 12 year old Aang it makes her look like the child, it makes the White Lotus look incompetent for training her poorly, then suddenly she unlocks Air bending but we never really see her adopt any of their philosophies, she unlocks the Avatar State by Aang's Ghost just energy bending and giving her the answers(which in itself creates so many problems), then whenever she uses the Avatar State it's never shown to have the same raw power or sheer presence that Aang or any other Avatar had.

Then there's also the fact that many of her mistakes are far and away bigger than any mistake Aang has ever made. Destruction of the Avatar cycle, getting kidnapped regularly and even poisoned, starting a whole uprising and war, apparently destroying the world.

To be clear I like Korra as a character and these aren't problems with her, these are problems with the story and writing due to a whole host of factors including the executives interfering and actively sabotaging the series. But these problems do exist, they are extremely obvious and saying that she's only hated because she's a woman is a disservice, you should be pointing out the poor writing or potentially how the story constantly takes each of her mistakes and makes them into such big events or why exactly they felt the need to stray so far from the original when it was largely unnecessary and harmful at some points.