I just feel like Rosalina wasn't play tested very thoroughly in this game. This was something I feared with Ultimate - with a character roster so large it was bound to have some characters who just weren't given as much attention. Rosalina is one of those characters that's just sort of there in this game, in all respects, but when it comes to competitive gameplay especially.
Aside from her hitboxes feeling inaccurate to her animations (i.e deceptively short or thin), there's a lot of questionable design decisions all around.
For one, I think they didn't realise how much the global mechanical changes hurt Rosalina at base. Like, they could have ported Wii U Rosalina one-to-one with the only changes being the global ones, and she would already have been more tamed. While directional air dodges are good for her, the nerf to shields and the nerf to consecutive dodging hurt her considerably. Especially the latter, which I feel hurts her more than any other character. Rosalina needs to use rolls to adjust her position without moving Luma when detethered, but doing so now comes at the cost of potentially sacrificing your intangibility frames if the enemy breaks through. That's a huge nerf to how her basic playstyle works already.
But they for some reason thought it would be a good idea to give her tons of other unnecessary nerfs. The most annoying one to me is that Luma now returns to Rosalina really slowly when she calls him back from a detethered state when he's at low HP. This is totally unnecessary, because the whole point of this character is that Rosalina is better when Luma is around, and is more vulnerable when he's been KO'd. Adding this in between state where Luma loses a great deal of effectiveness on his own when he's at low HP is just adding insult to injury. It's not the worst change she got, but it's the most annoying one because it's something literally nobody asked for nor was it at all warranted. What's more, there is a loading screen tip about it, so they are likely never going to relapse on it.
I also feel a change like this at least warranted giving us a UI element that displays Luma's HP at all times, since other characters like Robin got something similar, but we didn't even get that. This was forgivable in Smash Wii U/3DS since they only started toying around with new UI elements but there is no excuse here since many other characters introduced in that game got this feature in Ultimate. For the record, Ice Climbers should have it too for the secondary Ice Climber.
Nerfing Luma's frame data was completely unnecessary since everyone else has gotten faster, he's already less competitive with other characters because of that. If anything Rosalina deserved to be made faster while Luma stays the same so she could still have a competitive neutral game against even the fastest characters. Also her new N-air is good but Luma needs to hit low during it, for some reason he hits around Rosalina's waist when he should cover her feet. The old Luma N-air hit low and made MUs against short characters more manageable but here, Rosalina struggles severely against shorter characters especially the really nimble ones. Meta Knight in particular is a disgustingly bad MU for her in this game, she legitimately can't do anything because of his size, speed, and range in combination, and Luma basically does not exist. I know some people thought that MU was really bad in Smash Wii U/3DS but the Ultimate version of this MU makes the old version look positive by comparison. At least she had a usable N-air with Luma against him there, and spammable rolls and air dodges. In Ultimate, it's dumb luck if she lands a hit on him and she'll spend the rest of the match being used and abused as Meta Knight's ping-pong ball until she's KO'd. It's seriously one of the most toxic MUs I've ever seen in a fighting game and emphasises everything wrong with this new iteration of the character in Smash.
And as has been said, Luma losing the steps in jab really hurts his versatility and our ability to finely position him. Taking it away when he's tethered is one thing, but he should absolutely be able to have those jab steps when detethered, there was nothing broken about that and was necessary for the move to work when detethered.
Beyond thinking about what tier she's at, I simply think she's less satisfying to play as in this game now. She still has her moments and there are some improvements like detethered Luma responding faster to movement commands and the fact he automatically faces the nearest opponent which is really useful in 1v1 since there's only one opponent. But overall they just really screwed her over.
I'm sorry if I sound so pessimistic but it's just a let down that she feels like such an afterthought. I'm not surprised, especially since we barely saw anything of her pre-release and they gave her such a pitiful showing in the World of Light opening (hell she only gets two spirit battles in World of Light IIRC besides her unlock fight and the placement of her unlock fight feels like an afterthought too), but I had some hope she would still have solid gameplay and would remain competitive. But it seems even that was overstepping.
The only real hope I would have left is that she ends up as this game's equivalent to Smash Wii U/3DS Marth (i.e character that was solid in previous games but feels broken and an afterthought in the new one, but gets fixed through patches) but due to the nature of Elite Smash which is what they are using for game balance and the fact this game apparently had community members involved in balancing, I'm not seeing that as likely. This game just doesn't care for tricky characters like Rosalina and more for the rushdown, unga-bunga. Rosalina could do rushdown better in the last game which is just bizarre considering how almost everyone else turn out.
Anyway thanks for coming to my TED Talk, better get out before someone gets mad at me wanting good things for a character I like again.