Mario's matchup against Diddy on default settings is really awful, except that Diddy's game falls apart against Mario completely if he throws away the first stock. If for any dumb reason you manage to get the first KO, you probably can win very convincingly because of Rage assisted combos and because hopefully if you survived to high percents, Diddy will have a much harder time comboing you into a KO move. Diddy does very poorly coming back from a stock deficit against Mario for those reasons, but in most situations he generally outclasses Mario. Can break out of his combos with his 3 frame U-air, is really short and thus hard to space aerials against, has a giant F-air that beats almost all of Mario's attacks head on, and his grab game is better than Mario's.
Vs Rosalina is okay. Not really favorable for Mario, but doable. B-throw often instakills Luma which is nice, but what's not so nice is getting juggled by Rosalina's huge aerials and U-smash.