Good as a tragedy - Bad as Joker reinforcment
Could the negativity come down to the force and icon people want the Joker to be rather than the movie being bad on it's own merit? Admittedly the first one didn't have a particularly assertive, nihlistic character either but rather "a victim with a gun" - altough that one atleast still lashed out. This one is just downtrodden throughout and regrets what he did.
Then there's the political angle, where some people feel that it was pandering to those critics of the first one that claimed it was endorsing toxic behavior. Maybe it did pander. Regardless, i think the movie was a good tragedy of Arthur Fleck's life.
Honestly i feel that these movies didn't even have to be Joker to begin with they could've just called them "Clown" instead. It was always somewhat forced.