Some "nerfs" might be buffs in disguise. For instance, less knockback on certain moves might lead to nasty combos... or they might not. Only practice will tell for sure.
I can already tell you that N-air can easily link into dash attack now, which is great.
I'm completely fine with the reduced knockback on N-air because all of Rosalina's OTHER aerials were always far better for KOs. However, that was a slight problem at low percents, as it made follow ups more difficult. Now, N-air is capable of creating follow-ups, unlike before. So I consider the reduced knockback quite the buff - Rosalina has a new follow up. She didn't need N-Air to be a reliable KO move since she has plenty others.
Seriously, the only real nerfs here are the damage decrease on N-air, and the Luma respawn timer. The latter is non-issue if you're good.
As far as other changes, her grab definitely has more cooldown, but grabs are not a central part of her playstyle so I don't mind. Luma also cannot attack during THROW animations, but can still attack during grabs and other states where Rosalina is mobile (which is fine because it was really hard to hit people in throw animations anyway - now I can focus on vectoring).
Up Smas also launches at a bit of an angle now. I'm not sure if the knockback is necessarily decreased, but either way, it's still gonna be a KO move (I tested against Bowser, he still touches the top of the screen when hit by Rosalina's uncharged U-Smash at 100% - which means other characters would still go right off).
Down Smash might have more cooldown but I am unsure. If it does, it's not a very drastic difference.
In contrast, she's gotten some straight up buffs that are actually quite huge - Uncharged Luma Shot has a shorter distance (allowing for more varied spacing of Luma), Luma Shot has a LOT less cooldown (meaning it's a lot less punishable), and Luma returns to Rosalina WAY quicker when called back.
I think the positives outweigh, or at least balance out the negatives.