I appreciate that you recognized that there was no source on the claim's article, but were still able to find the CinemaScore website and confirm the movie's rating.
After visiting the website, however, I wonder how Zee was able to determine that Joker 2 was in fact the first movie to have received a failing grade, as the website only allows you to search for movies, and does not have an option to see the movies in rating order, or by genre. There have been a lot of movies, so it'd be interesting to spend the time to search every comic book movie created to see the individual scores.
Nevertheless, I would believe this claim solely based on the individual reviews I've heard from people who have watched the movie.