I don't think she's like replacing any character or showing up substantially more than say Waluigi (barring Mario Kart 7)
She isn't, really. My main gripe is that she's getting so much attention from Nintendo when she has no real reason to continue showing up based on her sole major appearance. It's like Knuckles going on adventures instead of guarding the Master Emerald as is his sole purpose in life (I still like Knuckles, but dammit, Sega needs to remember what his character is actually about once in a while). Getting put into Mini Mario & Friends while Wario doesn't would probably bother me a little more if I cared about that game, but the fact still does irk me a bit (although it makes me more angry at Nintendo than at Rosalina).
"why are they putting Wario in all these games and shoehorning him into the US version of GB Bomberman? He was perfectly fine as an one-off villain!"
I see where you're coming from here, but Wario also didn't have other matters he has to attend to. That opened him up for developers to explore him more in Wario Land, which worked spectacularly. Rosalina has the Comet Observatory and all her Lumas to attend to, and I'm a little baffled that she has time to go adventuring in 3D World and whatnot.
Metal Mario is basically just there because they can retexture Mario and have another character slot, sorta like how Smash Bros. gets so many clone characters because they're quick and easy to make relative to a completely unique character. Dry Bowser continues to appear as sort of a bonus boss every once in a while for pretty much the same reason, as well as being a clone character in spinoffs.
As for Waluigi...well, I like the groundwork of characterization that he managed to receive, and I wish Nintendo would do literally anything with that, but so far he really hasn't done anything of value.