This matchup is pretty annoying, but it is somewhat doable.
In neutral, what I usually do is stand back and kinda force Rosa to approach. Throw lots of PK fires out. Ordinarily, PK fire isn't really something worth using a lot, since opponents can punish you for it and simply roll out of it. However, this doesn't matter against Rosalina since what the PK fires are really meant for here are to kill Luma. Once Luma's HP is all gone, the matchup gets like 10 times easier, and Ness can just combo Rosalina without fear. If Rosalina approaches with Luma, try going in and using nairs and upairs. It sounds stupid, but the priority and range of those moves will occasionally stop inevitable attack from Rosa.
In terms of recovery, Ness is kinda screwed. Try recovering from as close to the bottom of the stage as you can, if you have to PKT2.
Combo-wise, don't pummel, as Enhalo said, when you get a grab. Go for short combos, and try to do them as vertically as possible (e.g. d-throw>rising fair>upair as opposed to d-throw>fair>fair>upair) because otherwise Luma can combo-breaker you with ease.
Never go offstage. If you attack, always go for the small space in between the two. If the matchup is still too hard, just pick up Falcon or Yoshi or some character that has ranged combos