Well, this is quite an interesting matchup. I have only played one tounament Rosalina so far, but just by that one set I learned a lot about the matchup (side note about that one set: It was one of the closest sets I have ever played. It went to game 3 and on our last stocks Rosa spiked me, and I was lucky we were playing on Dreamland with its low bottom blastzone. Any other stage and I would have lost for sure. Ended up coming back and winning). I have to say that Rosa is one of our better top tier matchups, but still not in our favor.
In terms of no customs, Dedede is not a free win for Rosalina by any means. Luma is really valuable in this matchup because it can reflect gordos easily, prevent our approach, make combos much riskier, and take a gordo hit for Rosa if Rosa is caught off guard. However, Dedede can get rid of luma pretty safely if Luma is not protected or Rosa's shield is too small. Just a side tilt is enough to send luma into a tumble, and D-tilt, Bair, and smashes are good at getting rid of luma too. When luma is gone, Dedede doesn't have to play as safe and can get aggressive. Also, Rosa is large and light, and Dedede benefits from both of those qualities. Rosa can die very early to a solid read, or a gordo ledge trap, or just a Bair onstage, and hitting her isn't very hard, especially when Luma is gone. In terms of ways to deal with gordo, gravitational pull is great when Dedede is at long range. If Dedede is far away when he initiates gordo, often he wants you to reflect gordo so he can reflect it right back at you. However if Dedede puts out a gordo too close to you, it is better to hit it back before he can react. Dedede is quite vulnerable when he commits to throwing a gordo. Also if you use gravitational pull while Dedede is close, he can get in and do some free damage. When luma is alive. Most moves can safely reflect gordo, particularly Nair, jab, star bits, or Dair. When luma is gone, it becomes a bit more risky. Bair is good if you can get it, or reverse Nair, or Dair, but otherwise it may be safer to use gravitational pull. You can choose to ignore it, but if you get grabbed or hit into gordo, you can take a LOT of damage, so keep that in mind. Rosa can certainly capitalize on Dedede's lack of speed though, especially his air speed. Even without luma, Rosa can juggle DDD pretty well with U-airs, and punish landings. If Dedede is above Rosa, you can nullify most of his approach options with U-tilt or U-air, and approaching on the ground is very difficult for DDD when luma is around, so it is possible to just prevent DDD's approach. In terms of off stage play, Rosa's recovery is very vulnerable, and if you don't mix it up, DDD can kill you really early. DDD can also chose to set up a gordo ledge trap. I think that if you drop from ledge and GP, you can get rid of the gordo, but don't do it too much or you might get stage spiked. Otherwise U-air or star bits can hit gordo, or you can try to get past everything but that is risky. Dedede is hard to punish off stage if he just recovers low and uses his armored up B to get back to ledge, so it is best to just try to punish his get up option.
(Wow I wrote a lot) okay so in conclusion, Rosa has a lot of tools that can shut down DDD's options when Luma is around, but Luma is vulnerable in this matchup and protecting it is very important. Be careful off stage, know when to reflect or absorb gordo, and make sure you punish DDD's commitments and landings.
I rate 55:45 in Rosa's favor
With customs:
Custom Rosa is scary, and has some really nice tools against Dedede. Luma warp is scary, but if you use it irresponsibly Luma will die for free, so don't spam it! Mix it up! Irresponsible warps will singlehandedly lose you the match. Distant star bit is really good for reflecting gordo from a distance, and forcing DDD to approach, making. Luma even more valuable. Guardian Luma is a good substitute for GP, but I'm not too experienced with that custom so I don't have much to say about it. Customs certainly gives Rosa more tools to deal with DDD, but makes luma even more important.
On Dedede's side, Dedede Storm is amazing at getting rid of Luma, as Dedede can just float above Rosa, then approach with a falling Dedede Storm which can get rid of Luma even if you shield it. However, normally Dededes run Taste Test, so unless the DDD knows that you are playing Rosa before hand, he will probably be using this first match, which is basically a better inhale that can't Dededecide or spit you offstage. Dedede storm makes the matchup very interesting because Rosa really benefits from having luma with Customs, and if Dedede can get rid of it so easily, it makes the MU very volatile. Dedede will certainly counterpick into this custom versus rosa, but this means that he loses his command grab, so shielding becomes safer, but doesn't protect Luma. Dedede's other customs can include Rising Dedede, just giving you more reason to never challenge him off stage, and Bouncing Gordo, which is really unpredictable but does much less damage and can't set up ledge traps.
Overall I would say that Rosa benefits from customs more than DDD does, but if Rosa loses Luma too much the MU can become even or possibly in Dedede's favor, but in general, 60:40 in Rosa's favor
