Well, I decided to post my insight on the MU since I main both if any Rosalina players wants some info from me.
Lets start with Mario, first.
: Mario is a rushdown fighter with good mobility, good frame data, combos, decent edgeguard and projectile, jiggling game, weight, safe ko options, and the tools get rid of Luma. Back throw, d-air, Forward smash, and up-b are what he'll go for to split up both Rosalina and Luma to get rid of the star and rush you down. Also, you'll have trouble getting him off of you when he plays aggressive and trying to kill him because her smash attacks aren't safe. Up-smash may help not only to hitstun Luma to get rid of it, but as a jiggle tool which is good against you due to Rosalina being floaty and her landing options aren't safe unless she uses Lunar landing. Once Luma is gone, Mario can combo her well because she's a large target for it. Be careful using Gravitational Pull because if he baits you to use it for his fireball, then he can punish you since you still have end lag along with Luma if doing it close. Only do GP if he does his fireball at mid-range or he chooses to camp. Mario can give Rosalina trouble landing safely since she's floaty so do not d-air on Mario while landing on the way down even if the halo hitbox(or ring XD) trades with Mario's up-smash because he'll go for a power shield and gets to punish your landing lag for free. One more thing for jiggling, his up-air is fast and good for not only for jiggling to also combo Rosalina, but can also auto cancel out of Short hop to bait you for a punish. Mario has fast aerials(but f-air) and can mix those up with auto-cancel and reverse-aerial for short combos, unpredictable approaching, mindgame, and baiting so watch out for them and do your best to stop his approaches by reading them. Also, his aerials are faster than Rosalina's. His ko options are alittle safer along with up-smash, down-smash, and back-air, and they're good with rage because Rosalina is floaty, really light and a easy target to land hits on as well. Plus, Mario can live longer unless you get him off stage and try to gimp him. Rosalina's recovery is gimpble since she has no hitbox so Mario can gimp with cape if he reads it, b-air for a stage spike, Fludd(not that useful for gimping, but just in case if he places you in a bad position or for spacing), f-air if he reads you, and n-air. If he reads your air dodge and di with his forward-air out of his down throw from the ledge at 60% least, then you'll dead so mix up your di and hope that you survived. Ledge guards are useful for him to kill you with his down smash and down angled forward smash. One more thing, with his ground mobility, he can mix his approaches with fox and dance trots, go for pivot grabs, tiles, forward smash, and up smash so keep a eye out for his mix ups, guess correctly, and shut his approaches while keeping luma safe.
Now with the Mother of stars.
: Rosalina's range, superior edgeguarding game, and jiggling game can be a huge threat for Mario. Use jabs to not only snipe down Mario's approaches since he doesn't have a good front approach, but also Luma's jabs for him(or her) to punch through the fireballs. Forward tile and down tile are good against Mario because their range can out-space and shut down his approaches along with Luma for the ride so use those. Also, Short hop n-air and b-air are useful for zoing and spacing against Mario along with Lunar landing for auto cancel. Jiggling game is more dangerous with Luma against Mario because he can't challenge your disjointed moves due to his low range aerials to get out of it so use up-tile, up-smash, and up-airs to jiggle him. Keep him in the air, above you if you can as possible, and start jiggling him with your dangerous up-airs(along with Luma) because like I said, he can't get out of them since your up-air is strong and disjointed, and all he got is air dodge which if you read it, then free damage for you. You can also combo him with your up-airs, get early kills with it(even without rage), and frame trap him with Luma's hitbox for more damage. Up-tile and up-smash are good anti-air moves against Mario's low range aerials. Lunar landings are useful for out-spacing him, getting small combos, baiting, and mindgame. Even though her up and forward smash aren't safe because of end lag and without Luma on your side, they can still kill Mario if fresh since they're disjointed with Luma, used to out-spaced him, and even kill early(along with rage) well as you don't just use them for him to get a easy punish on you so use pivot smashes to add options for yourself. Down smash isn't as strong as her other smashes, but the back hit can kill, used for damage, out space Mario, and to ledge punish the plumber. If you use GP on Mario's fludd, than not only you can act out of it after sucking in the water, but Mario will have to charge it up and gets nothing out of it. However, be careful using it off stage against his fludd because of the end lag, and it will be easier for him to read and gimp you. If you space your aerials correctly, then you'll out-space him first, but his aerials are still faster, though. Rosalina's edgeguarding is stronger than Mario because you can kill better off stage. Down-air is the biggest threat because it's disjointed and not only she can spike him down out of his low-ranged up-b, but can also knock him far with Luma's hitbox. Plus, that hitbox can get a early kill as well. Back air can be added as well for another gimp option. She can ledge punish him with down-smash, down-tile, down angled forward smash, and down-air on Rosalina's hitbox. You can ledge trumiping him for a back-air if you can. Neutral b for zoning and spacing, just don't charge it all of the way up and let him cape your Luma right back at you. He can't cape your star bits so out space you and mindgame along with Lunar landings.
Stages: Ban Lylat cruise because its her worst stage because of the stage tiling and it can mess up her recovery. Pick Battlefield because while he can combo you more and get more set ups, Rosalina can use the platforms to harass Mario by jiggling him more with up-airs, give him trouble getting off of platforms to get to the ground with up-tile, up-smash, and with along Luma as well if you guess his landing on the platform. Also, use SH n-air and b-air on Mario when he's on a platform for mindgame, pressure, and jiggles as well. Pick Delfino Plaza and Helberb the most because while Mario can get early verical kos with up-smash, up-air, d-air, and up-throw due to the low ceiling, Rosalina can be also get some earlier verical kills with the low ceiling from her up-airs and up-smashes, and can jiggle the crap of Mario with platforms almost the so she does better at those stages than Mario, including Battlefield thanks to her tall height and disjointed range along with Luma. Dreamland is ok for both of them and its the same as battlefield, but the wind to mess up her recovery. Duck hunt, both can well since Mario can't get stage spike kills, Rosalina can use her up-air under to harass Mario and even get a early kill, but he can live longer because of the blastzones.
Customs: Shooting star bit will be your to go, but look out for the cape, though. Guarding Luma to block Mario's incoming attacks, mindgame, and out spacing him. Use teleport Luma because he can't cape the star, and you can use the teleport for set-ups and mindgames. Shock cape he will use for another ko option and to knock Luma away even if you shield. Fast fireball are useful against Rosalina so don't use GP because its useless. Scalding Fludd if he wants to because even if you shield, the water still hurts Luma.
Results: 1+ in Rosalina's favor because even though
can rushdown Rosalina, combo her, ko her early, has better frame data, edgeguard, and has the moves to get rid of Luma,
can give Mario problems with her disjointed range, jiggling game, superior edgeguarding, and snipe out his approaches with Luma. So, she's really a big threat to Mario and can cause big problems for him along with platforms because of his low range, weight for her combos, and predictable recovery.
As for customs, its 0(or even) because Mario's shock cape and Fast fireball can help him get in on you more and get rid of Luma. Scalding Fludd is a option for Mario to space, mindgame, and can also damage Luma even if you shield.
Have fun with the MU, Rosalina players because its in your favor and as long as you out-range him with your disjointed moves, mindgame, jiggle him hard with up-airs, edgeguard him, keep Luma alive, and avoid serious punishes from Mario, then you're do fine so do your best against the plumber.