I don't play Kirby, but if I wanted to improve my Kirby, then this is what I think I'd do:
Generally speaking, you should practice your movement and ability to transition from one state to the next (assuming you aren't playing against people).
Practice your ground movement (particularly Dash-dance, wavedash, etc) in combination with d-tilting or other pokes (even if d-tilting nothing to practice,) crouching, shielding, grabbing, wavedashing OoS, and shffling out of shield mixed into it.
Practice your platform game by practicing your means to drop through platforms, running off platforms into aerials, etc.
Practice your pivot up-tilt into turnaround jump bair/uair, particularly out of movement and/or defense (Crouching/shielding). Especially practice your crouch into up-tilt.
Practice your overall zoning game of bair/fair off various positions in the ground.
Practice your air and recovery game. Practice sweetspotting; rising fair jumps; aerial turnarounds; vertical shifts with jumps and fast-falling; weaving and retaliating with attacks (fair, uair, or bair); ledgedashes or similar ledge get-up options.
If you have another person to play with, practice your combo, grab, gimp, and fair (shield pressure) game.
-Combo game is almost entirely up-tilt into uair or bair, so that's easy.
-Grab game: practice tilts out of poor DI higher percent d-throws, immediate b-throws (and sometimes follow-ups from poor DI), jab resets on missed techs from d-throw, combos on poor DI from up-throw, d-tilt tech chases, platform tech chases, etc. Also practice command grab (inhale), using new-found projectiles like lasers/needles/pills effectively, and the occasional follow-ups from mashing buttons after being spit out.
-Gimp game should be self-explanatory. Know when to wall of pain with bair, when dair is a follow-up for the meteor, when d-tilt will reset the situation with them off-stage, when D-Smash will do a similar think, and how to use your multiple jumps to perform a pseudo-float on opponents to force them into situations where they can be hit with bair due to the threat of bair earlier. Practice getting back to the stage from low gimps, especially off bair and dair.
-Fair pressure game should be self-explanatory, practice getting all three aerial hits in, then landing and trying to get the landing hit for a shield poke.
Also practice general spacing, mind-games/baiting/playstyle-control, knowing your hitboxes and how far they go, knowing your interruptible-As-Soon-As frames so to improve speed (idk how much this applies to Kirby).
