Quiksilver could help but he doesn't post much, same goes for DRN.
I have no experience in this MU but I somehow have a winning record against Zinoto and Player-1, so I guess I'm credible maybe.
Just be patient and space properly. Diddy has an amazing defensive game (good shield and banana is one of the best out-of-shield options in the game, if not THE best) so you have to be super careful with everything that might hit his shield. Sideb2, dsmash and bair are all safe on shield, I guess uair as well. Everything else WILL be punished by a banana (jabs, tilts, fsmash, usmash, upB, sideB1, lasers from too close, DASH ATTACK)
If he doesn't have a banana, nair is safe when spaced and retreated.
Be careful with lasers and sideB in general cause they have a lot of startup and they only hit from a distance where Diddy can hit you with a banana.
Never dash attack unless you're like 100% sure it's going to hit Diddy.
Normally if the opponent lands with an airdodge, you dsmash them for it. Diddy can fair you rather than airdodging so he can avoid the dsmash, cause it'll hit you before the dsmash comes out. If you notice this starts happening, use run away --> pivot sideB when you think he's going for fair (or just shield --> dtilt, but obviously sideB is a better punish)
Good Diddies will condition you to shield due to the threat of bananas, then sideB you when they know you're going to shield. So don't sit in shield too much. Or, sit in shield, but be prepared for the sideB. And don't try to nair his sideB kick.
If there's a banana in front of you and Diddy picks it up with a dash attack, you can dtilt/grab the dash attack before it hits you. Learn to SDI dash attacks as well (up/behind/up+behind)
Learn the infinite.