I read all the bad reviews before seeing the movie. I saw it anyway because I didn't believe the reviews. I doubted that Phoenix would invest himself in a bad film. I was right. It was a great film.
So, it got me thinking why the poor reviews? Okay, I admit I am no expert but here's my thoughts...
This origin story is the most realistic of the comic genres that I think it scares people into giving bad reviews to deter others from watching it. Why would they want to deter people? Well, the Joker's past life as a severely abused child, institutionalised and then humiliated is a story more common than we care to admit. The possibility of someone relating to the character is high. I think people are worried that it could trigger violence is my theory. It's the only explanation for giving this thoroughly thoughtful film such a bad review.