Kirby ***** DK, that's not a joke lol.
For KOs generally just choose your spots and plant your giant punch. Otherwise, spam low backairs quickly and watch out for CC dtilt or worse, CC uptilt or even worse, getting outprioritized by the uptilt. F-air is good to throw in for a mindgame, because Kirbs is generally bad at dodging things.
Kirbies are going to play gay spacing games and occasionally eat you and grab you. Their grab range is goot. Back air has the priorities but you want to be FAST with it and hit with it lots. Don't worry about missing Kirby doesn't punish you lols.
Oh and spot upairs if he goes to high, they kill at relatively good percents.
Why is this matchup bad? Play a Kirby that knows how to spotdodge. You really can't do anything amazing, that spotdodge is too good. It's like playing monkey vs. Peach, only without ... all the other gay stuff that peach does. Also, weirdly enough, all of Kirby's B moves are good against you (down b for edgeguarding, up b as a projectile, side b combos into uptilt kinda, neutral be eats you until you quit smash)
^^Ed: Above note on kirbycides, generally they don't go for the backthrow because it's too easy to wiggle, they'll use the upthrow.