Lol this battle was actually a joke Buu was downplayed so much.
They ignored that the punch used to crack Popstar was only 200 megatons of force showing that celestial objects in the Kirbyverse are less durable than IRL ones and applying out physics to the kirbyverse would be pointless.
Kirby's speed without the warpstar isn't even hypersonic (he needs jet Kirby to achieve mach 5) Buu should have blitz stomped him in an instant.
According to Death Battles own calculations SSJ2 Goku's maximum output is roughly
1 sextillion megatons, since SSJ2 Goku = Majin Vegeta according to Death Battle that's what Vegeta's maximum output should be the same and Fat Buu regenerated from that. There is nothing in Kirby's arsenal that comes even close to 1 sextillion megatons of force.
Also Kirby's 'crazy' strength feat where he threw a 3.5 ton monster at mach 73 speeds, is comparable to Tao Pai Pai's feat where he threw a roughly 1 ton pillar at mach 15 speeds. So let's say that this makes Kirby 20 times stronger than Tao, not bad but Buu is literally billions if not trillions of times stronger than Tao. So Kirby's feat isn't all than impressive.
Also Buu is ass durable as he wants to be, he can make himself tough enough so bullets bouce off him like any other Z-fighter.
And so what if Kirby 'tanked' a planet sized explosion, any Cell saga character could tank the explosion of Namek, do they beat Buu aswell?
Also the reason Kid Buu attempted to tank the spirit bomb instead of teleporting away from it is because he was certain he could deflect it. He was right but he didn't forsee Goku getting his power back.
Not to mention Death Battles hypothetical Buu saga SSJ4 Goku would be 2.5X faster than light, Super Buu should be comparable if not stronger than him, but he wasn't used. Death Battle had no problem using powerscaling for the white power ranger, but they refuse to use it for DBZ characters.
Buu easily could've absorbed Kirby or turned him into chocolate.
All in all it's like this Death Battle was made to give DBZ fans the middle finger. Buu should've stomped Kirby with ease.