Sorry, I think I went too extreme and gave no actual advice whatsoever. As a Kirby main, I think I can help.
1. He is pretty light. I don't know how is your playstyle, but I've passed through a barrier like that: Your attacks are not killing! As he is that light, he obviously dies much earlier than many characters. Despite the high percentage of the opponent, there is also the rage mechanic (when the opponent has 50%+ damage, its attacks have stronger knockback than usual), which allows them to win against you easier... Also, straight-out attack with your strongest attacks, and try to get them with the quickest move possible (Down-Smash is pretty good in that regard).
2. Play the most passive-agressive you can, and mix-up your approaches (as PeejsterM said, there might be staleness of your attacks). Try to chase them with the Dash Attack as they run away, and read them with Smashes (Hammer might work in case they just approach with no attack, but it is very, very situational).
3. Use your mighty recovery, and try to gimp your opponents too!
4. Ducking under some attacks might be useful as a defense, since Kirby has quite a low crouch. His hurtbox shrinks very much, and as his Down Tilt can make them trip, you will have the opportunity to finish them with a deadly blow if you are quick enough.
I also would recommend training your DI to keep living in higher percents.