Ah, Diddy Kong. I would say that prior to patches 1.0.6 and 1.0.8, we lost this matchup BIG TIME. Diddy's Banana , F-Air, and Peanut Gun made approaching a tricky enough matter. But what really wrecked it for us was the good old Hoo Hah. Kirby being one of the lightest characters made it even easier than normal for Diddy to get a grab and send up into the blast line with an Up Air. Between really good frame data, speed, better aerials, projectiles, set up tool in the banana, and his broken kill set up, it was not a fun time for Kirby. The only notable edge we had was that a Diddy forced to recover with the Rocket Barrels was a relatively easy gimp and the Monkey Flip was predictable. Aside for a better off stage game, we really didn't have much going for us.
But as time went on, things changed. Nintendo saw how broken Diddy was and dropped the nerf hammer HARD. Diddy's frame data was toned down slightly such as having more lag on missed grabs, his Down Throw no longer confirmed into solid kill moves and Up Throw was slightly toned down, and his damage output as whole dropped big time. But most notably though, they nuked his infamous Hoo Hah. Without rage, it won't kill until very high percents and the Up Aerial has more end lag, hurting its combo potential.
Meanwhile, things have only gotten better for Kirby. Our jabs have become more solid, our Inhale is now viable, and we now have legitimate combos out of F-Throw that even Diddy isn't immune to.
How does this all mesh now? Well this matchup went from being pretty bad, to largely even. Thanks to all of Diddy's nerfs, we now last longer in the battle, and thanks to our buffs, we can rack up damage more easily and get his Peanut gun. It's not a matchup changer, but can come in handy, especially since Diddy loves to camp slow characters out at times. And now that we stand on more even ground, we get more opportunities to knock the monkey off stage and prey on his poor recovery which we can easily do with D-Air and Down B. Most of all though, his Hoo Hah is a shell of its former self. At most he can get some damage off with it, but he can no longer get an easy kill on us with it as we now have much easier time floating out of his combos.
Of course, can't sleep on Diddy. His continued, high performance in the metagame prove that while he's no longer OP, he's still a force to be reckoned with. His banana still gives him guaranteed follow ups in his still good Smashes, his speed still lets outmaneuver us, his F-Air can still keep us at bay, and his damage output is still better than ours. Also, while he has lost a crucial kill option in his Hoo Hah, he can still trip us with his bananas and bop us. We on the other hand still don't have a reliable kill option. (I think D-Air to D-Smash works relatively well on him but it wouldn't compare to having a reliable kill throw in Up Throw).
I personally feel that the matchup is still in his favor, but the gap is WAY smaller than it used to be. Diddy's strengths against us have been massively toned down while our strengths against him have become more practical. It's now a perfectable do-able matchup for us as long as we don't underestimate his less effective, but still potent strengths and options. And if we get good kill setup or throw, it'll probably become even.
