i don't think it would make a difference, because you get her anyway after you finish mirror mode.
It probably wouldn't make a difference because when you unlock rosalina I'm pretty sure you also unlock another kart so if you did use it, it wouldn't help or ruin your run.
Me and Rhodechill argued about this on one or two other threads for 3-4 months on and off. Basically, you could, in theory, have a Galaxy save, get Rosalina early and butcher doing a single Mirror cup and get an A rank but still "unlock everything". I don't know for sure, but with neither of us testing it, he certainly knows for sure and is adamant about it!
It'd help if you believed rosa to be faster and wanted to use her earler in your run
but yeah, with the galaxy save file, it allows you a buffer where you can a rank one of the mirror cups and be fine
in 100% do you need 3 stars for all cups in all Grand Prix
No, just at least 1 star with the exception of the hardest cup in the game for Rhode which he could not stop arguing with me about.
