I've played this matchup a good bit. It's not fun, but that's mostly because Rosalina isn't a fun character to fight for anyone.
I think it's a slight disadvantage, and it'll likely get better as we get comfortable with her pokes. Her hitboxes are obnoxious and do a good job of defending in places we want to attack, but she's pretty slow about getting into range without going nuts with U-Smash, which is fairly risky for her. It's a matchup where we kind of get to call the shots, which is nice. The trouble is that she's allowed to move backward at virtually zero risk to herself, so the matchup is slow-going, but if you're patient, it's not so bad an issue.
Luma is annoying, but if you can avoid stray D-Air/U-Air hits, you shouldn't die at especially low percents, so it's mostly a matter of getting through the wall of hitboxes. Jab is annoying and safe for her but doesn't net her a whole lot. Rosalina being tall and light is quite useful for Jigglypuff; D-Air is great to land as it separates Rosalina from Luma, does good damage, and allows you to pressure afterward; just make sure to mix up your retreat if she shields it, since if she blocks all of it, that U-Smash is coming your way. She's probably the easiest character in the game to D-Air -> Rest combo, so keep that in mind.
If you can react to Rosalina's N-Air, your Forward-B, U-Air, Dash Attack, and even Down-B can work to bop it. Forward-B is great for hitting Luma, and Rosalina's lack of speed is often covered by excessive shielding; I've probably broken significantly more Rosalina shields than any other character as Jigglypuff. Watch for those opportunities.
Her recovery is fairly exploitable, especially on stages without platforms for her to retreat to; stay jumping near the edge, as you don't get a lot for chasing her far, and Rollout will actually help you punish if she decides to go up and around you. Her aerials are big and annoying, but she's not gonna N-Air while recovering, and U-Air/F-Air aren't difficult to airdodge/work around and punish.
Take your time in this matchup. I'll say 45-55, though maybe I'm just well-versed in a matchup that Rosalina players generally don't know well? Dunno.