I'm sure it was considered "must watch" but that meant streaming for many people. Must watch for Captain Marvel required a theater visit. My guess, based purely on instinct and how much I enjoyed Captain Marvel is that it probably got a 33% or so box office boost from the Endgame tie-in.
I'm sure it was a factor for many people overall, but the quality drop between the two Captain Marvel movies was not nearly as bad as the box office implied.
Lots of factors could have influenced it. Tying it into a so-so TV series didn't help. I liked the Kamala character and chemistry between the actors, but the show didn't make me want to see her in a movie.
There's also the post-Covid factor where movies seem to hit it huge, or bomb, with nothing in between. If a movie "clicks" people will show up, but if a movie doesn't hit that "must see" threshold, people won't go.