I was not surprised this did poorly. I don't feel it was bad marketing, the lack of actors to market the film, or superhero fatigue. I think it simply has to do with needing to watch the
Disney+ shows to be interested and not enough people have done that. It's the same reason Ahsoka didn't do as well as Book of Boba Fett, Mandalorian, or Obi Wan. You had to go into Ahsoka knowing the storyline and why it was set up the way it was.
Disney has got to stop doing this with their movies and shows. The cross pollination of MCU and Disney+ shows is only going to lead to more box office flops. You are asking way too much of the audience to care about these new characters who they may have not even seen before. If this was a straight Captain Marvel stand alone movie, it would've stood a better chance. I enjoyed it but I already watched WandaVision and Ms. Marvel first so I was familiar with everything. If this was a continuation of Carol out in the universe helping after the blip with some nod to the Multiverse or Kang then it might've done better.
If you look at most of the sequel movies, they aren't dropping in what appear to be established characters and relationships into the sequel. Winter Soldier, they build up Cap and Natasha's relationship. You already have Cap's close relationship with Bucky established. In Civil War, they add in a new character, Black Panther, but give you his backstory. They don't drop Black Panther in and just assume you know who he is. Guardians 2, more building on existing relationships from the first film. Iron Man 2, they build upon the existing Tony and Rhodey relationship. The only one that broke that mold was Dr. Strange 2 and they have an already established character in Wanda to play the villain. That said, the writer of DS2 didn't even know how WandaVision turned out and that was evident from how she was written in the movie and the sudden maniacal turn she had.
I think the decision to take this from a Carol movie to a Carol + Kamala + Monica movie was a serious miscalculation. It felt more like something that should've been on Disney+, not a theatrical release. I think Marvel needs to really take their time and figure things out with the direction they are going to go. I know some are hoping Fantastic Four changes the current slide but I feel like that's a story whose time has passed. They're also going to have to pivot away from the Multiverse, both because of Jonathon Major's legal issues but also because it's already stale.
