I don’t even think it’s a good idea for such a last-minute, little-known character to play a big role in Endgame. For her to steal the thunder to defeat Thanos from the original characters after their 10 years of build-up would be grossly disappointing. At least let her help the Avengers some but nothing more than that.
While I do agree that it’s more of an issue with the character than the marketing, the marketing is really overselling her. Trying to proudly claim that she will be the face of phase 4 (or is it 5?) before the BO results are even in is pretty overly zealous, isn’t it?