I think a lot of that depends on how much of an “important role” she really ends up having. You could argue Ant-Man, another hero introduced one movie before a big team-up film, had an important role in Civil War. He ended up being one of the key players in the airport fight and dominated a decent chunk of the entire scene. However, he was never the focus of the movie.

To use an equivalent example with The Last Jedi (which I personally enjoyed, but I know was pretty controversial), if you didn’t like Star Wars 8, you probably won’t like Star Wars 9 either. However, Endgame is supposed to be Avengers 4, not Captain Marvel 2. If you don’t like Captain Marvel, there should still be plenty more stuff for you in Endgame, probably for the majority of the runtime even.

Even if Captain Marvel does poorly and isn’t considered very good, I don’t think it will put a significant dent in the sequel to probably the biggest movie of 2018. Just like how Thor: The Dark World didn’t put much of a dent in Age of Ultron.