Backlash is both justified and not.
On the one hand, you have misogynistic, racist and islamophobic jerks who can't bear the idea that a Pakistane muslim girl is getting enough attention to get her own show that will try to treat muslim culture and beliefs respectfully.
On the other hand, the show is not perfect. I've only watched the first episode and some scenes later but honestly, this isn't that great. Outside of the teenage movie tone, the plot is not really exciting, action sequences aren't different from anything you've seen before and the characters are all very cliché save for Kamala. There's also the changes done to her powers and her origins, wich pissed a lot of people (especially since the implied reason why they changed her powers was because they're planning on adding Mr Fantastic and the idea that a muslim girl got her powers changed to favor the introduction of a white man later is problematic for some people), not to mention their adaptation of islamic lore with stuff like Djinns being...not like they're supposed to be, wich is its own buttload of problem.
I've also noticed people who praise the series's exploration of muslim culture are non-muslim white for the most people while many muslim (including here) are actually disappointed by the treatment of their culture. Overall, there are many people who like this series just for the representation and even some who say "Respresentation is more important than quality" wich can be enraging for many.
And on another side, you have people who are just tired of Marvel being everywhere and just decided to hate it precisely because it's successful and they've had enough of superhero stuff getting attention over and over and over. Or those who simply don't like teenage like stuff, wich is the whole basis of this series. Or people who are pissed about the series's success when series like Moon Knight or Loki, wich they actually liked, were less successful. And then you have those who don't like that people are saying this series is as good as The Boys or even that it is better than The Boys.