I was just surprised she's on the poster at all. I know that a lot of the characters on the IW poster didn't have big roles (Wong, Shuri, etc.) but the point of that poster was to squeeze every character that they could on there to emphasize the fact that it was the biggest undertaking Marvel Studios had done (at that point, at least). The Endgame poster is keeping it simpler with the original six, plus the other important characters like Rhodey, Rocket, Nebula, Scott, Carol, and Thanos of course. Okoye could have a significant role to play, but out of all the characters on the poster, she's the only one we haven't seen in any trailer or TV spot. She may not have a big part in this, especially since T'Challa isn't around (for most of the movie, at least).