Some things that can help people on the Diddy matchup:
I played around with bananas and tried some things.
Picking up banans:
Aerials: First I thought about how to pick up bananas if they are in front of you and Diddy is behind them. You could do a down B, but most of the time (at least when I do it) the Diddy will use side B or jump and use an aerial and you get hit. Soo... just pick them up with aerials. Nothing new I know... it's easier to pick them up if they are in front of you. You can sh bair or uair to catch a banana, and you can then even do another aerial while holding the banana. Although full jump dair or sh/fj fair could also be nice.
If you catch a banana with an aerial and it ends quick enough, then you can do another aerial (if it also ends quickly, you can do it over and over...).
If you wait too long, you will throw the banana, even if the aerial ends quickly.
If you double jump, you can't do another aerial, you will always throw the banana.
This works with DKs uair, bair and nair.
http://www.youtube.com/watch?v=rq9PcOYrzOU
DACUS: You can also use your (poor) DACUS to grab a banana on the ground and usmash at the same time. Maybe it will help sometime... (if the banana is in Diddys control he might think he can land on it safely, but you catch it and usmash him...) Though this is probably really sitational.
http://www.youtube.com/watch?v=rq9PcOYrzOU#t=0m36s
Use your SA to get through bananas:
Another thing you can do is using your SA on the bananas. Especially if they are on the ground (because it's not always so easy to time it right when Diddy throws them).
Practise this.
You can use your grounded upB and giant punch on the bananas and you won't slip if you touch them during your SA. This probably helps
a lot in the matchup if you have the timing down.
http://www.youtube.com/watch?v=rq9PcOYrzOU#t=0m44s
Throwing the banana:
Insta-throwing: Do you guys know about insta(nt)-throwing? If you catch a banana with an airdodge and press A in the first few frames you will catch the banana and throw it immediately. You throw them much quicker than if you would catch them with Z and then throw it as soon as possible. It's somewhat hard to do with DK so it probably needs some training.
http://www.youtube.com/watch?v=rq9PcOYrzOU#t=1m3s
Momentum cancel: The item throw that ends the quickest is the dthrow for DK. If you have a banana in hand (or a peanut) and get hit and might die so you want to momentum cancel throw the banana DOWN to regain control (then DJ/ upB...).
If you're used doing bair it won't be much worse, because the item bthrow is the second fastest... although the dthrow is the best (because you also don't have to worry about which direction you have to press).
Glidetossing: I think DKs glidetoss is really bad. Even though it goes far you have so much lag before you can do anything again. Which most of the time get's you punished... and you can't really follow up with much. Maybe throwing the banana forward is somewhat okay. If the Diddy is behind you you could throw it back while moving away although I don't know how this would be useful other than racking up a few %... maybe the item dthrow can surprise the enemy (he might shield) and you can buffer a standing grab (in the other direction, if you are somewhat near to him). If you did the "pivot" grab you can dthrow the Diddy, but he won't have enough knockback to be thrown into the banana, maybe Diddys that don't know this will be scared and you can get in another hit/grab, I dunno.
http://www.youtube.com/watch?v=rq9PcOYrzOU#t=2m2s
Grab releasing into a banana..
Chaingrabbing: I thought about some grab release stuff and it seems to work. If you have a banana in your control and it is on the ground and you manage to grab Diddy, cargo him and go to the banana. You have to face the same direction when you grabbed him or else he can somewhat control where he is released to avoid the banana. AND you aren't allowed to go as near to the banana as possible, because if he DIs away he will go past it. You can grab him again and go to the nana again, you can chaingrab him like this (he will always get 6 % damage if he gets released from the cargo).
Although if he doesn't have much % it probably won't work, because he'll be released too quickly.
This is best to do during mid %s (30-60?), and watch out, because the banana disappears quickly...
You can probably only do it 1-2 times. It's probably best to grab release him once and then pummel him and bthrow him or cargo uthrow/dthrow him away.
http://www.youtube.com/watch?v=rq9PcOYrzOU#t=2m26s
KOing: TO FINISH DIDDY just use your dsmash after the cargo release. Yeah, that's right, you can grab release to dsmash Diddy Kong to KO him!! Of course he has to land on the banana like with the chaingrab.
http://www.youtube.com/watch?v=rq9PcOYrzOU#t=2m44s
If the Diddy knows about the cargo release stuff he could wait until the banana disappears to not be hit by it, but if you're near a edge you could throw him away from the stage or cargo stage spike him so he's in a bad position if he doesn't try to brake out... if you plan to hit him with a dsmash don't chaingrab him first so he can't wait for the banana to disappear more easily... to make him try to brake out grab him and pummel him once, then cargo him, he probably inputted some commands to shorten the cargo time...
EDIT: The banana disappears REALLY quick, much quicker than I thought. If Diddy has >60 % and doesn't press any buttons he won't be released while the banana is still there. Even if you grabbed it as soon as possible and grabbed Diddy as soon as possible. This means if the Diddy knows about it pummel him before you cargo him to make him brake out, then cargo him if you hear him mashing buttons. Even if he stops he probably still mashed some while the cargo time was and he'll be released quicker...