That's a bit of a stretch. Nobody was pulling a "why is Joker white ugh" card out.
I don't think it's a stretch.
A few years ago this movie would have simply come out and done it's thing. Maybe some incel group would have attempted to co-op the movie after the fact, but they would have been scoffed at. Their would be talk about how some of the elements are problematic, but that would simply be one of the discussions about the movie after movie had already been absorbed by the culture. The movie would be allowed to breathe.
Now though, everyone has to find something problematic even before whatever the thing is out. Yeah sure we're going through some difficult times currently. but the reason this movie is a target is in part because it's lead is "white enough" for the general population to think that. So because of that it gets taged as already maybe having problematic elements before it can even do anything and from there its a race for the tiwtter woke to pat themselves on the back as sesensing that a movie may have some elements that could be twisted by shitty idealoges. And from there it spins out of control.
I doubt the real thought process behind the movie was "make an incel movie" or make " a white male shunned movie". It was probably just make a movie about the joker who is normally a white male and most of his themes when exploring his background deal with someone who is shunned by society. So while those themes do inherently have incel appeal, I doubt the movie makers were ever trying to make a movie for them. Sure its a movie made by white dudes and it probably has some bias. So what? Does it's themes have some issues in 2019? Sure, but if anything your normal American culture would have simply seen this movie as "hey it's about the Joker and I hear it probably takes from that The Killing Joke thing I heard about". Most of the people would'nt have read much more then that. Some internet nerds would maybe take it a little too serious, but hey they also did The Matrix and that hasn't changed the legacy of that movie. These nerds would probably miss the angle of the movie that the Joker is bad and thus made fun of like any idiot who probably thinks Thanos had it right.
But instead the woke media had to get ahead of the curve and make you know "hey this movie has some themes that are bad and you need to agree with it". Which is so funny, because thats what incels want. They want you to be against them, they want you judge them and confirm thier shitty worldview. The alt right wants the media to be against them. They want to say "well gee why can't a movie about a shunned white guy exist no problem" "If the movie was about a minority there would be no problem". These people already have a complex about being prejudiced against. The controversy about this movie probably only fueled that and most of it was based on opinions that had already made up thier mind before seeing the movie. Opinions that decided the movie had this problematic theme before even judging the whole picture.