The movie has a black female lead, a trans-male best friend, who's mother supports his gender identity, and another teenager that is the child of an interracial couple. There is also a Native American social worker, an Indian student, and a couple of southeast Asian supporting cast members. The movie discusses jailing children, and the negative effects that can have, and it straight out condemns the private prison system. And it also discusses Hell in a way that doesn't paint everybody trapped down below as eternally evil. If you've ever suffered any sort of bigotry, misogyny, or racism, you are probably starting to see why this movie was called "bad". It had nothing to do with the quality of the movie, it was about the quality of the audience that got uptight about it.
I will admit, Wendell and Wild had some slow pacing, and some weird violence (slight spoiler, somebody gets drowned), but that is the type of tale it is. The lead character is still suffering grief and survivors guilt, the pace fits the depression in her soul. Honestly, if you can put away your privileged biases, it's a good movie. I'm just sad it got slandered so hard just because it tried to be real about life.
