Being a background character in one non-sports game and a secret fanservice character in one other really doesn't qualify as "major". She ranks between Waluigi and the Blue/Yellow Toad right now.
She was basically the main character of Galaxy (the "Mario saves Peach" plot was completely overshadowed by Rosalina's side of everything in that game), had some relevance in Galaxy 2 (was essentially the narrator, and used her Cosmic Spirit power to help you if you wanted), and has been playable (and usually objectively one of the best stats wise, meaning a lot of people use her - MK8 being a prime example right now, in which she is undoubtedly the most used character online) in many of the biggest Mario games at this point, be it Mario Kart, 3D World, or Mario Golf (in which you had to pay extra to get her at all). She's also got this tendency to be this coveted, difficult unlockable in games, as if playing as her (or indeed,
having her chill out on your ship in Galaxy 2) is supposed to be some kind of honour. The 3D World dev team have also been quoted as saying she is "well known".
Also she's basically Nintendo EAD's mascot - Mario and most characters in the main series are created by Miyamoto, Rosalina wasn't, she was created by Nintendo EAD, specifically by Yoshiaki Koizumi if I'm not mistaken (who I know wrote the story book). A lot of people will say that Miyamoto created her, but in the Iwata Asks interview for 3D World, he does admit to directly opposing her creation, with the reasoning being that "we already have a female main character in Peach, we don't need any others". So yeah, EAD's mascot.
If this isn't enough to be considered "major", I really don't know what is. She's a more modern major character. If Greninja is major enough for Smash, Rosalina definitely is.