I alluded to it at some point last night but I want to acknowledge that I feel like people overvalue exactly what role
plays in the mainline Mario series, particularly at the time of her inclusion. Since I saw someone just say spinoffs are largely irrelevant to character selection, I'm going to apply that logic here as well if it's such a non-factor. How important is Rosalina?
There's Super Mario Galaxy, her debut and her defining role as a "major" character. She's Mario's guide and has a distinct role in the story. Galaxy 2, Rosalina's function is replaced by Lubba. She appears in the end as a 100% completion reward, but otherwise has no influence on the game. In all fairness, the "Cosmic Spirit" exists throughout the game to assist with levels and looks a lot like Rosalina. But these are decidedly very minor roles.
Then there's Super Mario 3D World, where Rosalina makes her debut as a playable character. This is her sole playable appearance, and she is a post-game unlock. I think it's important to note though, that Rosalina was locked in before the fact. The roster proposal was finalized in May of 2012 and she is distinctly credited as being from Super Mario Galaxy. She was revealed for Smash within a month of her playable debut in 3D World. It's safe to say this was not a large factor in their decision to include her.
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All of this to say, how different is Rosalina from these one-off Zelda characters or Kirby characters? Well, the clear difference is the factor of Mario spinoffs to keep a character in the public conscience, which is not the be all end all but it helps her recognizability. Still, I can't imagine this was the determining variable in adding her to Smash... and even so, Hyrule Warriors exists now if we really wanna play that card. So does Star Allies or Kirby Fighters.
I think Rosalina being here is perhaps the best piece of evidence that these kind of characters are perfectly viable, the fates just haven't aligned for any of them to be here yet. Or maybe Sakurai hasn't been as uniquely inspired as he was to make Rosalina a puppet fighter. Bottom line, if we want to criticize the validity of Skull Kid or Midna, or Marx or whoever, we gotta look at the character most comparable to them on the existing roster.