I don't see how Rosalina has been consistently debuting in low tier iterations of spin-off series'.
The only one everyone agrees is subpar is Ultra Smash (and even then it's just a content thing, the gameplay is still solid, best it's ever been I hear).
Everything else is subjective. I liked Mario Party 10 (though she technically debuted in Island Tour).
But I actually personally hated MKWii, and Rosalina's portrayal in it is probably her worst or second worst too.
Also those voice and character "derailment" arguments while I was gone were exceptionally silly.
Saying Rosalina's character was "derailed" is a complete lack of understanding of what the term means. The only Mario characters to ever suffer legitimate character derailment are Bowser (they can't decide whether he's just a lovable "not necessarily evil" villain or a vicious Ganondorf-wannabe, which usually results in downplaying other characters to make him look better. The former is better as it has consistently led to him having more of a personality), and Peach (originally touted as the wise ruler of the Mushroom Kingdom possessing some kind of magical power that Bowser wanted, turned into a complete "dumb blonde" stereotype. They occasionally re-rail her though).
TV Tropes agrees with me too.
Rosalina has not been derailed as she has never exhibited some kind of outrageously major out of character behaviour. She's only had small instances (MKWii, Mario Party 10) and none of them even define her portrayal in those games, plus they aren't story driven to begin with, so they need not be taken seriously. Rosalina was just a much more diverse character than most Nintendo characters from the get go, so there is a lot they can work with. Some games might hone in on specific traits of hers more than others, but that's not a bad thing and meets complete sense (for example, it makes sense to hone in on her motherly side in Smash since Luma fights with her, but you can't really do much with it in 3D World since Rosalina flies solo or with the other playable characters there, so her competitive, tough, cool, collected, and confident side is emphasised much more here, as it fits the tone of the game. She had all of these traits in Galaxy, though).
With all that said, I want to share something:
View attachment 98293
This screenshot has been shared here before, but PushDustin (owner of SourceGaming, they translate a lot of Japanese stuff, like interviews with Sakurai, he's a mod on this site too), casually translated this when I posted about it in another thread. Rosalina is saying "Well then, let's go to a fantastic extra world together!".
Just a little tidbit I thought I'd share. I guess Rosalina is going to take the player to another world at some point in this game? Maybe she is our gateway to dream events in the story mode.