I mean people had the same negative reaction when Chris Pratt was announced to be voicing Mario, then people heard his voice and it was pretty good. You can't even tell it's Chris Pratt's voice. From the clips we heard her Rosalina voice sounds good, so it doesn't matter about the real life actress.
And yeah, Rosalina is probably the most popular Mario character after the original cast. She eclipses everyone. Pauline is only starting to get really popular after Odyssey and DK Bananza.
I'm surprised Yoshi didn't get an announced voice actor? Maybe Yoshi won't actually talk at all and just make the dumb Yoshi sounds from the games like a Pokemon? I'm used to the Super Mario cartoon where Yoshi spoke english.
