Hey Mario & Sonic Guy! I'd love to give some input in my perspective as a Jigglypuff main to help you guys out.
Default only: +2
Some may be surprised by this since Jigglypuff is such a low tier character, and many would think that the score should be the highest, but Jigglypuff should not be underrated. Rosalina is combo meat for Jigglypuff because she is vertialy huge (Jiggs combos horizontally most of the time), and Rosa can die at 50% to rest, which is actually pretty early compared to other characters. However, since Rosalina is thin, it is harder to rest her.
Gimping is also somewhat easy for Jigglypuff, since Rosalina is wide open during recovery. It is important to put on pressure with Rosa while recovering by using Luma, or spacing up airs, down airs, or back airs depending on the situation (Fairs will have too much lag if it isn't obvious enough). Usually, Jigglypuff will be below and to the side of the stage to either QUICKLY chase you with a fair or two killing you (it's faster than you think), or to guard the ledge for a ledge guard, or to start a combo by ledeg guarding. The safest option (against me anyway) is to recover BACK ON STAGE avoiding the ledge. It could be safer since Jigglypuff is generally slow, and will probably not be able to chase you down for a punish. If you're facing a mediocre Jigglypuff player who doesn't quite have the concept of ledge guarding down, than perhaps you should go to the ledge, but a good Jigglypuff is a DEADLY Jigglypuff!
This matchup will probably play out as a bunch of defensive play since both characters play very defensively, but Jigglypuff has somewhat of an offensive game, and could come at you. When I play Rosa, this is what I want!! Rosalina has combos out of grabs and spacey moves, and it does out space Jigglypuff. The best offensive option for Jigglypuff is fair becuase it can separate Luma and rosa, and with little landing lag, Jigglypuff can grab with bad reaction time by the Rosa player. In this situation, you'll want to grab! Down smash or forward smash is also a good option becuase poking sheild with fair isn't safe for Jigglypuff, and many do that. Going back to grabs, Down throw to up airs won't work as well because of weight, but down throw to fair can still work for awhile. Also, Jigglypuff will be superior in the air, so try to stay on the ground unless you get Jiggs in the air by an up air or up tilt. Jigglypuff has poor landing and jumping frames, so punishing with a down air can be safe, but while landing she crouches, so many punishes can whiff or miss.
Jigglypuff also can only jump back on stage, making recovering horizontally unsafe for her as we can spike Jigglypuff with down air, or hit her to the side. Jigglypuff isn't the only good ledge guarder in this MU!
My final will be that you can probably get out of some combos because of nair, dair, or jumping because Jigglypuff can't really chase opponents well, on top of bad frame data to hit you with another move. This is just a theory though. Don't quote me on this one!
Because you outspace Jigglypuff, and have powerful moves, you guys do win the matchup. however, spacing aerials is harder, and you're generally unsafe in the air! Please please stay on the ground unless you know you can get a down air roll read or punish or combo in the air. Don't be too aggressive.
Customs on: +3
The same MU, but you guys have a better option or two now! The star bit projectile will be badish for Jiggs becuase of her floaty ness, and jumping and landing lag. She can only sheild the move, or air dodge it, which is Only one option on a domain. I'm pretty sure Jigglypuff can't crounch under it, so that's good for you guys right?
Jigglypuff doesn't have good customs, but many puff players use pound blitz instead of pound. This will change toe MU, but in YOUR favor. The combo out of it is usually up air, and thigh we might be able to string an up air or two at the beginning of a stock, you guys should be able to dair us. This makes landing fairs much harder for Jiggs sometimes, and that's her second best thing to string off of ( behind dair, which we usually can't land
).
Hope this helped
I wrote this kinda quick so let me know if there's errors.