You've read article after article on this site written by better people than me about this film, so I'll skip the hyperbole and just get right to the facts.
Cinemascore is a market research firm that has been surveying moviegoers for their first reactions to films for over 35 years. Audience members have been historically generous to movies, as the exit polling occurs on the first Friday of release and captures those people who were most enthusiastic to see the movie. This has led to scores that aren't always an accurate representation of the film's quality. While a handful of films have been given an F over the years, most films generally fall in a range of A to B-. A movie generating a "C" Cinemascore is generally regarded as a failure.
Movies that get very good or very poor reviews sometimes see Cinemascores that are opposite of their reviews, as the audiences grade the film on a further curve. A movie that is poorly reviewed might see their Cinemascore come in higher, a reflection of audiences saying "It wasn't THAT Bad." A recent example of this is Entourage, which got an A- Cinemascore despite poor reviews.
Likewise, critically acclaimed movies sometimes get lower scores as the glowing reviews bring in audiences that might not have normally seen the movie. A recent example of this is Mad Max Fury Road, which got a lower than expected B+ in spite of universally positive critics reviews.
So now that you know how to read the Cinemascores, here is a list of the lowest graded Comic Book movies that I could find on Cinemascore. I could find only one other Comic Book film, The Spirit - that was graded as low as the Fantastic Four.
Only movies that were graded B- or below are listed. Because I know some will ask, Catwoman, Elektra, Green Lantern, Judge Dredd (1995) and Steel all received a "B". This backs up what I was saying about audiences responding as much to critics as the movie. I couldn't find a score for Supergirl.
Hellboy: B-
The Hulk: B-
League of Extraordinary Gentlemen: B-
Punisher: War Zone: B-
Son of the Mask: B-
Batman and Robin: C+
Ghost Rider Spirt of Vengeance: C+
Jonah Hex: C+
RIPD: C+
Superman IV: The Quest for Peace: C
Fantastic Four (2015) C-
The Spirit: C-