Mario kinda scares me, that cape...
Other than that he's no different from captain falcon, you can just combo him with your nanerz like any other guy. Mario's not hard to spike, and diddy is probably hard for mario to edgeguard.
lulz, indeed. The cape is epic.
Fighting Diddy is different from any other character. Basically, you're not fighting diddy, you're battling his bananas. Grab them first, and keep them away. If he has one, know that's as bad as a chaingrab; possibly worse. Camp him with fireballs, he's searching for an opportunity to start up a combo game with them. Try as hard as you can to keep that chance from him and rack up damage at the same time. Shift to the offensive ONLY when you've created an opening with one of your fireballs; moving too early gives Diddy the perfect opportunity to rack up easy %.
When you have created your opening and start up your combo, uair combos and utilt chains work well. End your combo or w/e with something to reset the current battle position to neutral, with you on the defensive. A Bthrow, Fthrow, Dsmash, Fsmash, or Nair does this nicely.
When he
doesn't have any bananas, the fight is yours. You have priority, KO power, Combo game, and a good jab game to work with. Your aerial game is better as well. Bair and Dair are your best friend; Dair especially for grabbing nanerz.
Gimping Diddy is a little tricky as well. Caping his SideB is easy because cape has priority and range over it. His upB can be caped as well during the charge and as he's moving toward the edge. Your ideal edgeguard would be to Fludd his sideB instead of caping because it's much safer (SideB can gimp you VERY easily if you screw up your cape timing) and keeps him from really moving forward. Here, all he has left is his doublejump and his upB; a cape would screw both of these. If he does his upB, try and get in more than one cape to mess with his aim.
Diddy edgeguards you well too. Peanuts screw up your second jump, Mario's easy to spike, and he has his sideB. Don't underestimate his dair, especially. It's very quick and has excellent priority and range.