I'm not saying "OMG SHULK AND PAC-MAN DECIMATE ROSALINA", but they are the best characters to use against her IMO by virtue of the fact that pretty much every other character in the roster is either A) completely shut down by her, or B) they may not be completely shut down, but they have no particular counter or escape from Rosalina's most deadly options, such as U-air.
Shulk is pretty much the perfect match for Rosalina since he has a weapon that outranges Rosalina regardless of whether or not Luma is present, and due to his Buster and Speed Arts, he can combo her pretty well despite her "anti-combo" light weight. He pretty much wins neutral, though Rosalina has the advantage in the air and definitely off-stage. It's kind of difficult to get him into those positions though, and you are reliant on slipping Luma into his "blind spots" (Shulk's will mostly be using their N-air and F-air against you at ground level due to the auto cancel frames on those moves, leaving very few opportunities to counter him. The moves have distinct blind spots, though, albeit they are hard for Rosalina to take advantage of). However, with customs on, Rosalina wins the MU very easily, as her customs fix all of the problems she otherwise has in neutral, and Shulk's customs are flat out bad and gimmicky so he's best just keeping his defaults for the most part. I also think it isn't as hard for Rosalina if platforms are present, as it gives Rosalina more openings to catch Shulk out on.
Pac-Man is a decent character to use against Rosalina IMO because he is the only other character in the game that can rival Rosalina's superb stage control. He has many different projectiles to play with that are hard to read, and he can place hydrants and trampolines about the stage, and even get around with the Power Pellet, which gives him invulnerability once he actually consumes the pellet itself. He also is the only character with an EASY answer to Rosalina's U-air. If Rosalina attempts to get him with U-air, most of the time he can answer that by simply throwing down a hyrdrant, which pushes him up slightly, out of harms way, and goes through Rosalina's U-air. If the Rosalina player is foolish enough to try this while near the top of the screen, this is guaranteed to KO her from as early as like 40%-50%. Rosalina CAN potentially fake Pac-Man out by baiting the hyrdrant and then GP'ing it close enough to him so that it hits him, and KOs him if the percent his high enough, but it only really works if the Pac-Man isn't prepared for it. With that in mind, Pac-Man is the only character that can safely avoid being juggled by Rosalina with a near completely consistent rate. Once again, however, Pac-Man does not fare as well with customs on, and I think Rosalina has an advantage even with defaults on any stage that isn't FD/Omega.
So yeah, while these characters are far from impossible for Rosalina to face and Rosalina still arguably has an advantage against them, they are not MUs she can take lightly, and you have to know what you are doing to face good Pac-Man players and Shulk players.