I guess I'm not bad with Rosalina, but I'm definitely am not good with her. I suck at effectively using Luma, so it ends up dying quickly.
Rosalina is actually just horrible in Ultimate so this is not you.
Even if you perfectly understand how Luma works, he still gets knocked off way too easily because everyone else is so strong now (the 1v1 multiplier doesn't help either) and he has bugs with his A.I. Rosalina also has super buggy hitboxes now. A lot of Ultimate's base mechanics also hurt her a lot.
And that's before getting into the plethora of targeted nerfs she got. As a devout Rosalina fan (not just in Smash but in general), and as someone who found her the most fun character in Smash Wii U/3DS and was at least solid with her (never entered a tournament but a lot of people were begging me to and I did play matches with some tournament players), Rosalina has become one of my least favourite characters to play as in a competitive setting in Ultimate. She's easily the most nerfed character coming from Wii U/3DS IMO, and has just been completely butchered if you ask me. It's not even about her tier placement for me (I think she will be low tier at best in the long run, if she isn't patched), I straight up think they rushed her out the door and they didn't really playtest her. She's not the only character who got neglected on this huge roster, but she's one of the best examples of a neglected character on this roster. Even if you master her, you'll never be consistent with her because of the buggy hitbox issues and buggy Luma A.I.
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Aside from Rosalina, Sonic has a lot of weird changes that I still haven't fully adjusted to. I'm not sure if he's strictly worse but he's very different. Also I just can't play him online, my GSP with him is pitiful. However I played a lot of matches offline as him with my brother and was doing a lot better.
Rosalina and Sonic were my most used characters before so Rosalina being butchered and Sonic taking getting used to has led to me doing a lot of experimentation. I have a weird aptitude for Samus in this game. Weird because I wasn't very good with her in other games.