I don't think Rosalina is broken by any means, but she quickly became my main in Sm4sh, and I have far more success playing with her than I do any other character.
With Rosalina, I win against roughly 90% of my opponents in For Glory, whereas with other characters I've been trying to make secondaries (Ness, Zelda, Link) my win rate isn't nearly as good.
I'm not sure how much of this is that Rosalina is just plain better, and how much of it is that Rosalina is just a really good fit for me, but I can't seem to get any other character to even being close to as good as my Rosalina.
Often times when I play Rosalina in For Glory, I can just feel that my opponent is a much more skilled Smash player than I am (the bulk of my Smash experience before this game was casually playing Smash 64 with my friends)... I say this because these players are using advanced techniques with perfect success, like always teching properly, and doing some crazy maneuvers around the ledge, perfect timing on their air dodges, etc... I'm assuming these people have been playing Smash consistently for years, as opposed to me who played hardly any Smash at all for the last 5 or 6 years.
And yet... when I play Rosalina, more often than not I am beating these players who I can tell are much more experienced than me. These are people I would have no chance of beating with any character other than Rosalina.
What makes Rosalina so good for me? The dash attack is incredible. Her aerials are freakin' amazing. They have priority over pretty much everything, they have long range, they last for a ton of frames, they're powerful, and even when my opponent air dodges it barely matters, because Rosalina can quickly do a follow up aerial.
Her roll is fabulous. Her down smash picks off roll spammers at will. Her grab game is also pretty good.
I use the Luma toss very selectively against skilled opponents, because I find otherwise Luma dies way too easily, and I find the benefits of a separated Luma marginal compared to having Luma close to me (more powerful Smashes, blocking projectiles, and a quick escape from many grabs and combos).
All of this is to say... I would like to think that Rosalina is not broken. But for me personally, I find that Rosalina is easily my best character. Ultimately, I feel like Rosalina will be at the very top tier of Sm4sh, but not broken. Time will tell though. If Rosalina keeps winning the major tournaments in Sm4sh, I wouldn't be absolutely stunned to see more people calling for a ban.