is not bad for sure. It's just that she belongs somewhere around the middle of a massive mid tier, so it's understandable that people aren't too interested in analyzing the minutiae of her mediocrity.
She has a solid advantage state between her excellent juggling and edgeguarding (is there anybody better than Rosa at juggling?), so there's some reward in winning neutral. Her neutral game is fairly decent, with the best boost grab in the game, star bits to disrupt the opponent, and good pokes with bair. Tech like lunar jumps open up a lot of traps and setups as well for her. She struggles with projectiles a bit because they'll hit Luma even if you shield them. Rosa is held back, more than anything, by how squishy Luma is.
He dies to everything in this game, and when he's dead, you get put in disadvantage very quickly (SoRo is the 4th worst character in the game, right after SoPo, Nolimar, and Luma himself). Being in disadvantage is extra bad as Rosa, because Luma will often die right as he spawns next to you.
Still though, she's definitely underrated. She can be played effectively with multiple playstyles, which is huge, and her matchup spread is pretty solid. I'd say she beats or goes even with most of the cast, but loses to most of top tier. However, other than Olimar, I think that all her losing matchups are very doable. A lot of Rosa mains say she beats Inkling and Snake, so that's something.
If Luma was given some sort of meaningful survivability buff (More HP, knockback resistance, the ability to recall him after her gets launched offstage at higher HP, etc.), I firmly believe that Rosalina would become a very threatening character, and borderline high tier.
Another issue with the character in my opinion is that she's stupid hard from a technical point of view (Peach hasn't got anything on Ultimate Rosa) so it makes her very difficult to learn, and so results and top level gameplay are lacking. When a lot of the better characters are easy to pick up and do well with (Wolf, Lucina, Ike), there's not much of a reason to pick up Rosalina.