This is our worst "top tier MU", imo and in all honesty, for a lot of reasons. I personally don't like ratios, but simply explaining this match-up from my perspective should be enough to show how bad of an MU it is for a puffball. It's winnable for Kirby, but he has to play MUCH harder than the Rosa player.
Kirby is a bait-and-punish character, primarily, meaning that he gets most of his stuff based upon making bad moves or pressing bad buttons. Rosalina, with Luma in play, has no bad buttons on the ground or in the air. Sure, Luma can get hit by a punish, but it just gives Rosalina a free hit to clock Kirby with, and the damage does stack up fast. So long as Luma is out, Luma can be a range extender and a free bean-bag to take a blow for Rosa, allowing her to perform a huge punish back, and since Luma's well-being is priority #1 for Rosalina, this is an easy thing for her to exploit. Then there is Star Bits, which is basically a complete shutdown on Kirby.
Advantage-wise, Rosalina simply usurps Kirby. While Kirby has grown stronger, Rosalina and Luma are both already loaded with tools to KO Kirby. Luma's D-Air KOs Kirby at even the 20%+ range, and since her aerial mobility and floatiness allow her to stay in edgeguard play, it is hard to near impossible for Kirby to safely get back on stage. Kirby cannot ever be above Rosalina because her and Luma's U-Air are just as deadly. Kirby basically has to stay grounded, all while Rosalina floats around and does whatever since even a successful punish for Kirby can go right into a potentially stock-ending punish for Rosalina and Luma, since her Smashes are surprisingly very good for KOing, especially with Luma in play.
Disadvantage is Rosalina by miles: Launch Star is fantastic against Kirby thanks to how fast she gets launched (no pun intended), and Kirby's just too slow in the air to do anything to save himself. Pretty simple. Rosalina just does not want to go too low, since D-Air can get her and force a stock, and going too high or starting launch too high can mean a Smash. That's not as bad as Kirby's disadvantage, as stated with D-Air or U-Air demolishing his recovery a good chunk of the time.
Kirby does have some things to look out for, even though things are stacked. Rosa does not want to give Kirby Luma Shot, since it allows Kirby to get rid of Luma very quickly; it goes through Rosalina's shield and can whack Luma and gives Kirby a decent ranged option (though it cannot be wantonly used since Rosalina's speed is good enough to weave and punish Kirby). Kirby's tilts can put Luma in a hitstun lock and whittle away at its HP, but that can lead to a free hit on Kirby. There's also Kirby's U-Throw, which thanks to the 1.1.4 patch, can KO Rosalina very early off of a throw.
Kirby's gameplan is to find a way to separate Luma from Rosalina as SAFELY (not as fast), because then and there Kirby can get good punish options; however, the issue becomes time. Rosalina can easily camp out Kirby and force him to approach, and if she has the % lead, Kirby has to take gambits and hope they pay off if she is in the lead, and then pray he does not get KOed for inappropriate or incorrect actions/punishes. Smart play and doing the above is the only solace for Kirby; otherwise, this is in Rosa's favor by a significant margin.