Spam UpB OOS and your already better than kirby.
People say "Thats all Bowsers got!" True, but at least he has something, what does Kirby have? Swallowcide?
-Bair/Fair/tilt walls/zoning
-An up-tilt that both combos and converts off many approaches (especially Fox approaches)
-Decent edgeguards
-Gimps
-Up-tilt combos (including kill combos on some characters and set-up edgeguard combos)
-Pretty good tilts overall
-A decent recovery when using rising aerial-jumps
-Multiple jumps alloting shifts in both height and falling speed for recovery & platform-game mix-ups
-An up-air that kills pretty well on a platform tech-chase (especially after an up-tilt) or after a few select combos
-Some tech-chase abilities (especially after a throw or an up-tilt)
-A pretty good jab reset iirc (definitely way better than Bowser's).
-The ability to cover an option on a tech-chase and return to neutral before the opponent can punish (Bowser has a pretty hard time doing this) thanks to relatively fast attacks
-A down air that's harder to meteor cancel than Ganon's and isn't realistically able to be teched
-A pseudo wall-of-pain that can lead into said down-air or an edgeguarding sequence
-A d-tilt with the right properties to set people up for more while edgeguarding
-A crouch that avoids most of what kirby can't crouch cancel until mid percents
-Plenty of character specific tech/strategies and/or mixups due to having more options that take less than 15 frames to initiate than bowser could dream of
-Legitimate access to wavedashing due to having an a 4-frame (same as Fox/Sheik) jumpsquat compared to bowsers 9 frames
-Normal landing lag that doesn't take 6 frames (takes 4 instead)
-A much faster "ramen noodle" as a result of said jumpsquat
-Faster overall
-A much better spot dodge (tied for best compared to Bowser's alone at worst [twice as long])
-Better rolls (Bowser is worst by a decent margin)
-Air-dodge more effective (while Bowser's is invincible 1 frame longer/better, it's gives him far less distance relative to how much hurtbox position has changed, especially given Kirby's means to do things afterwards much more quickly
-Pretty big grab range (just under Yoshi) compared to Bowser's 3rd worst in the game
-Faster and otherwise comparable grabs overall
-Access to jump-cancelled grab
-Knocked down by shine
-Not really combo food
-Access to plenty of great tools with swallow/copy.
--Short hop double falco laser
---Laser off-stage without dying / while being able to follow-up
--Short hop triple fox laser
--Sheik's needles off-stage without dying / while being able to follow-up
--Better at pills that Dr. Mario
-Kirby is not only much harder to projectile camp, but can projectile camp right back after a swallow (which isn't as hard to land as it may seem due to a large/weird hitbox and b-reversal)
-Many characters who don't have a special move worth copying can be put into bad situations (and I think even a few combos on some) from the swallow/spit (which also does like 15% damage (character dependent, 19% on bowser).
-Can throw caught items (Turnips/bombs) at about 200x the speed of Bowser (Bowser takes like 30+ frames to throw an item while grounded, no clue while in the air)
-3 (possibly 4) matchups that Kirby actually has an advantage in (including Bowser)
-And of course the swallowcide you mentioned
--Alongside ill-advised kirbycide throws
Against Bowser, you can just play extremely lame/respectful and he'll never stand a chance -- he basically requires that you roll towards him or shff(no-L) attack him in shield. Respect his shield/ledge-clinging, space his fair, and don't put yourself at risk for one of his very limited reads, and there's no reason to lose to Bowser with almost any character. You have to actually play a legitimate neutral game (though obviously Kirby's neutral is far weaker than the great majority of the cast.)