Diddy > Ness
Diddy pros:
Peanut gimp F***s with Ness' recovery
Banana gimp does the same XD (Bananas interrupt PKT2)
racks up damage VERY fast
naner lock > spike (with proper spacing, it is guaranteed against Ness)
If you techchase (attempt a fair) a landed PK fire, he WILL DI out and Diddy hump you. Don't let him.
Ground game > > > > > > > > > > > Ness' ground game. Stay in the air (you shouldn't be on the ground anyway, silly Ness).
Grab release to dtilt? Not sure but I believe I've pulled it off. Someone test it.
Diddy kick can (and will) be autocanceled
cons:
Air game is inferior to Ness'
fB is predictable (it can go one of two distances, far, or short, both take the same amount of time)
PKT1 absolutely ***** Diddy. You can not use this move enough. It's too good.
Spike the predictable recovery.
You CAN single naner lock Diddy!!! Banana in hand: SH above Diddy, cstick down, cstick down, dash, repeat.
bthrow/bair kill~ 120. Too good.
But, in all honesty, this matchup goes to Diddy at least 60:40 in his favor. The character is just too fast and bananas are too sexy.
Protip: Sweet Jesus don't let him barrel cancel you (If he's charging his upB while you have a banana in hand, don't throw it. It will hit him, but not trip, and he'll send a barrel flying off in a random trajectory across the stage. They hurt).