This is definitely one of Rosalina's worst match-up in my opinion, and I've heard this from a lot of other people too. Sure, Cloud as a bad recovery, but it's not so easy to abuse with Rosalina. Sure, you can easily spike him with D-air if he recovers low, but if he recovers high it's a lot harder, he can use his Up-B over the ledge to protect himself with the attack and grab the ledge pretty safely. You can also try to go gimp him offstage, but Rosa's aerials are a bit slow so it's not as easy as with other characters.
The reason why Cloud is a bad MU though is because he dominates in neutral. He has a lot of moves with fast, powerful and huge disjoints that are really good to hurt and kill Luma quickly. Heck, even his jab puts Luma in tumble and can kill him near the ledge. So Luma is always dead, but Cloud's Limit Break is also ALWAYS fully charged, since it fills when he deals damage to Rosalina BUT also to Luma. Also, when he knocks you offstage or in the air, this always give him a lot of time to charge his Limit, since Rosa is so light and floaty it takes her forever to get back on stage.
Speaking of offstage, Cloud is also not bad at edgeguarding Rosalina. If you recover low with Up-B, he can go offstage and spike/stage-spike you with a D-air, hit you with a N-air, or stay on stage and deny you the ledge by simply sliding on it with D-tilt. If you recover high and he reads it, he can U-smash or Limit Side-B you and you're dead.
Cloud also limits the use of one of Rosa's best move : her U-air. It's really hard to juggle Cloud with U-air like any other character since he can use his D-air, which has a lot of range. It's like one huge disjoint VS another huge disjoint, and most of the time the attacks will trade (Rosa suffering more damage than Cloud), and even sometimes only Rosa will get hit. I've been killed once because I tried to hit Cloud wiht my U-air, but he used his D-air and it knocked me upward to the blastzone. So you really can't use U-air a lot in the MU, you have to rely more on U-tilt (which surprisingly always beats D-air) or other things like shield-grab to punish his landing. It can still be hard though, if Cloud autocancels his D-air.
Imo this is -2.5for Rosa with default moves.