I don't have much experience against Diddy, but I'll try to write something.
Character: Diddy Kong
Difficulty rating: 60-40
Overview: Banana spam all day. They usually go all offensive, approaching with glide toss to grab, banana into dash attack followed by u-tilt. But once they are forced into a defensive position, it's hard for them to regain momentum.
Pros: fast, good projectile (bananas), fast aerials and smashes, good juggling.
Cons: light, predictable and gimpable recovery, it can be hard for him to land killing blows and (I think) Kirby out ranges everything Diddy has, at least in the air. Oh, the bananas can be used against him.
Watch out for: bananas, all over the place; forward and down smashes, fast and kill; the occasional f-tilt, fast and can kill relatively early since it will be probably be fresh.
How to win: stay defensive, on the air most of the time, spacing f-airs and fast falled b-airs, trying to get him on the air where you can juggle him. Your smashes, mainly f-smash will be fresh since you won't be spending much time grounded, so you'll kill him early, and never waste a chance to gimp him, otherwise this match will take forever.
Spit out or Swallow?: you should probably spit out, because his peanut popgun is sub par and due to his bad horizontal recovery from below, spiting him under the stage is a good idea, and depending on the angle it can be almost impossible for him to come back, and because he has to charge his up-b before recovering, d-air is great at gimping.
What NOT to do: stay grounded, that's all I have to say.
Stages: Frigate Orpheon should be a good counter-pick, no ledge on the right side, platforms. From the neutrals, Battlefield should be your best bet, 3 platforms for you to avoid bananas. You should avoid FD, no platforms = ****, he has total control of the stage.
Synopsis: stay aerial, abuse b-air, don't miss a chance to gimp him, watch out for the bananas.