throwing bananas at Diddy is a bad idea. its very easy for him to regain control of a banana thrown at him, and DK's glidetoss is one of the worst in the game: it goes really far, yes, but (and this is a big but) he has a huge amount of lag from his throw. so basically what happens is, DK glides towards the enemy, completely vulnerable. a thrown banana is easily powershielded, or even just standard shielded, and then the glide can be punished very severely.
its a much better idea to Z drop bananas during shorthops, that way the bananas stay under DK's control and they stay close to DK where Diddy will have to approach to regain control of it.
another thing that may be useful is the following: if you grab a banana, you can still do smash attacks. when you grab the banana, keep A pressed. do not release the A button. then, you can use the C-stick to perform your smash attacks. i know this sounds dumb, but if DK can perform smash attacks, wouldnt it be possible to perform regular tilts? this is a legitimate question, if somebody can please do some testing to see what happens it would be a great help. if it is confirmed that DK can use his tilts and jabs (maybe even aerials? somebody test please) while holding a banana, imagine how much easier it would make the matchup. it would force Diddy to always have 2 bananas out, while only being able to use 1. "Thats great, but how am i going to hold down the A button and use my tilts at the same time?" What i'm thinking is that if it is possible, then you could set a button you dont use so often (for me it would be the L button) to attack, so that you can use that button to pick up items, and to keep it held so that you can use your thumb on the A button to perform your tilts. Like i said, all this is just speculation on my part, i dont have a wii, so i cant test all this stuff, but i think its important to know whether or not its possible.
OK, so all that info above was for dealing with banana camping/spam.
the rest is for dealing with Diddy without bananas. It's much easier, but its still not a walk in the park. Diddy's dash attack is too good. safe on shield, fast, hardly any lag and it combos into his usmash. downb could stop it, but its too slow. i honestly dont know what DK should do in this situation, my guess is upb OoS. Diddy isn't completely helpless offstage either. his recovery is predictable, but its also really really good. his sideb can grab us and put us in a bad situation, while helping him to get back onstage safely. bair will help a great deal in keeping Diddy offstage. he also has a blind spot below him, the hitbox for his dair is not so great.
the good news is that Diddy wont kill DK til he's well into 160+%. dsmash is pretty easy to see coming, unfortunately, he can combo into it with his bananas. fair will kill at about the same percent as dsmash, and he can also combo into it from his dash attack. fsmash is SDI-able, especially at higher percents where the first hit will send you further away from Diddy. 2 other moves to keep in mind when you're at death percents are ftilt and utilt. they will kill, and utilt is pretty fast and has a pretty good hitbox (Diddy does a little hop to extend his hitbox), so dont go above Diddy at kill percents or you'll be surprised.
DK is going to kill Diddy at ~100% or less. 9 wind will deal with him much earlier, which is nice. Dsmash is good since its so fast. uair works well also.
the key is patience. DK will kill Diddy earlier, but its more likely that we're going to be at high damage when we finally do get the kill. dont let Diddy get in your head or you'll lose even faster. capitalize on any mistakes you catch. downb spamming wont work against good, intelligent Diddy players. you can use the upb superarmor frames to avoid slipping on a banana peel on the ground.
good luck.
60-40 Diddy
vs. 
