well, the movie is called "In the multiverse of madness".... and if Feige wanted this movie to show one of those universes having mutants on it...... you gotta show mutants, you know.
And this whole phase (Wandavision/No Way Home/Doctor Strange) is about the multiverse and returns/alternate versions, so it makes perfect sense to me. Not to mention that to general audience would make the film look bigger and like an event.