CinemaScore polls moviegoers who have seen a film on its opening weekend. It's pretty hard to get a D this way - Fs are even rarer and almost solely given to really artsy dramas or "extreme" films. My understanding is that anything below a B-level is, when it comes to wide-release films, pretty much only goes to horror movies, which have VERY particular target audiences and expectations (those films rarely get to A-level). Prior to Joker Folie a Deux, the worst-performing superhero film was the 2015 Fantastic 4, which got a C-. Even The Marvels scrounged up a B. Family movies often get in the A/A- range. The lowest-scoring Star Wars installment, Rise of Skywalker, got a B+. (Last Jedi got an A!)
So Joker Folie a Deux performing this badly in the poll is an indicator that this movie isn't just disliked but loathed by just about everyone who got out to the theater. Superfans of the first film felt insulted by it (many many many of them apparently never figured out it was a musical), people who were neutral about the first were just bored silly by it (seems Harley doesn't get as much screen/plot time as you'd think; the back half of the film is mostly a courtroom drama). It didn't give superfans what they wanted, and it didn't work as something that appealed to general audiences. And while someone who was let down by The Marvels or just thought it was okay would likely just shrug and move on to something else, people let down this badly would very much be angry enough to mention it to friends, social media, etc....
