Rosalina isn't in the original version of the game. However, Nintendo did state during the trailer that there would be additional new content coming to the Switch 2 Edition of Super Mario Bros. Wonder, but that it would share more details at a later date. It certainly seems like Rosalina might be one of the surprises Nintendo was looking to keep under wraps until closer to the game's release.
Today's Nintendo Direct featured all kinds of announcements and reveals, from The Super Mario Galaxy Movie to a new Yoshi game to a Metroid Prime 4: Beyond release date and more. It also featured a fresh batch of amiibo releasing over the coming months for games like Super Mario Galaxy 1 and Super Mario Galaxy 2, Kirby Air Riders, and Metroid Prime 4: Beyond.