Oh god. If this movie is really going through, I'm really interested in seeing how they'll handle Toad's voice work.
Not really. Those guys have been there since the beginning, and they're all iconic and at the same time important for the Mario series. Rosalina's relatively new to the Mario series although she's really getting more popular with Smash Bros and Mario Kart 8 helping with that. At this point, I'd put Rosalina below all the characters you've mentioned and above everyone else (she's on par with Wario) in terms of importance to the Mario series. I'll just wait until next generation to see if she'll still be a staple to Mario games (Which I think she still will be)
Importance =/= Age (in real life or in game character terms, might I add).
Flickies (Sonic's equivalent to Toads) have been in the Sonic series since the beginning, and even have had their own game. But they are still nowhere near as important as more recent characters like Shadow, Rouge, Blaze, Silver, etc.
Or take Lucario - undeniably one of Pokémon's poster children. If Pikachu is the cute mascot of Pokémon, Lucario is the cool, serious one. He's fairly recent compared to literally tons of other Pokémon. But even still, he's way more important. He's had huge roles in the anime, and had a big role in X & Y too, and is always showing up in promotional material.
How important a character is for a franchise is more defined by the specific roles they have in the franchise, both from a real life marketing perspective, and in-game perspective.
Canonically, Rosalina is basically the most important Mario character in that she is basically the universe's equivalent to god, and all of the other characters would not have existed without her, and certainly would not have survived Galaxy's ending without her.
In real life, Rosalina is considered a hugely marketable character, hence why she is appearing in all of these games, and hence why her Amiibo is exclusive to a particular store (i.e. she is considered popular enough to bring income in to a specific retailer - like Lucario). Even in Japan, with the playing card pre-order bonuses for MK8, Peach wasn't even the queen card to Mario's king card - it was Rosalina.
And unlike Wario, she has appeared consistently in the MAIN series (spin-offs are irrelevant, anyone can show up in those), while Wario has only appeared in a remake of one. Rosalina also convinced many people to buy 3D World so close to launch (similar to Bayonetta being unlockable in Wonderful 101, as it was also revealed before release via Twitter, I believe). Not to mention that we even have Baby Rosalina in Mario Kart now, yet we still don't have Baby Wario or even Baby Bowser, despite the latter two actually existing pre-Mario Kart.
Of course, Wario is definitely an important Nintendo character with his own games, but he's not much more important as a Mario character than say, Link. This is in contrast to Yoshi, who remains an important and staple Mario character despite having his own franchise.
Rosalina hasn't got her own game yet, but she's much more consistently an important part of the main Mario games than even Yoshi is these days (though I still consider Yoshi more important than her to Mario right now, alongside Luigi and Mario). However, Rosalina is just as important as Peach and Bowser, and is more important than Toad (and no, Captain Toad does not make him more important, that game isn't being made because of the character, but because of the ability to re-use assets and flesh out a gameplay style people liked in 3D World - I'm saying this as someone who loves Captain Toad, too).
Not only that, but Rosalina represents a willingness to change in a franchise that is often criticised for not doing so, probably more than any other character, which I also think makes her very important.
