As the post-credits stinger on the first movie that announced her involvement alongside Cable proved, Domino is just as big a part of the Deadpool sequel as Josh Brolin's Cable.
Unfortunately, Cable has taken the lion's share of the marketing so far (which is SOMEWHAT inevitable, even if it's not fair), when in reality, he needs less of it. His fans have already waded in to their necks, but Domino is more of a challenging sell.
Considering the first film's difficulties with the supporting cast (aside from maybe Negasonic Teenage Warhead), fans need reassurance that the rest of the cast around Deadpool are going to be an improvement. And Domino will definitely be a big hope on that front. It'd just be nice to see some more of her.