1. Like I said, if you want to take that seriously, you also have to take things like the 30 mph speed of the Warp Star seriously. Kirby never displays that kind of strength outside of that one minigame, so we should assume he's not actually that strong. If everyone on popstar had planet crushing power, the games would be much different. No cherry picking.
2. In that game, Kirby also struggles to keep up with King Dedede on foot (which makes it hard to believe he could keep up with Buu), and the warp star takes quite a while to catch up to Metaknight's war ship which certainly does not go at the speed of light. No cherry picking.
He does travel accross the Galaxy, but we have no idea what the size of his Galaxy is, or how long it takes him to travel between planets. The planets he travels too are all pretty close to eachother. Also, none of them have a sun, or any discernable orbit, so I don't know how seriously we want to take this representation.
In the anime, Gotenks can fly around the Earth 9 times in about 5 seconds. So it takes him roughtly half a second to travel around the Earth, between 1/3 and 1/4 the speed of light. Buu is faster than that. Kirby is not dodging him on foot. Buu also copied instant transmission from Goku.
3. Still far stronger. At any rate, nothing in the anime (that I'm aware of) directly contradicts the manga regarding Buu. And, they didn't even use Buu's strongest form anyway.
4. Games where Kirby can hold 5 powers- Squeak Squad. Games where Kirby can only use current ability- Kirby's adventure, Kirby's Dreamland 2, Kirby's Dream Land 3, Kirby's Return to Dreamland, Kirby's Dream Course, Kirby's Canvas Curse, Kirby Superstar, Kirby Triple Deluxe, Kirby 64. So, picking the one game where Kirby can hold five powers would be the definition of cherry picking.
And there is no indication whatsoever that he could store hypernove. In Triple Deluxe, Hyper Nova does not follow the same normal copy rules. It doesn't get hit away from Kirby, he can't take it between levels, etc.
5. Why was he supposed to die? Where exactly are you pulling this stuff out of? All of Dreamland was turned into Yarn, and nobody died. Kirby has never shown the ability to hold Hypernova for more than a level. And with Hyper Nova, it takes Kirby about 5 seconds to suck up a tree about 5 times his height (40 inches) that is not resisting. He can't suck up bosses until they're rendered unconscious, so he can't suck up Buu.
While we're at it, Kirby cannot survive an exploding planet. What he survived was the vines exploding on popstar. (https://www.youtube.com/watch?v=6qaND3cCxoQ 9:00). It sort of looks like an explosion, but it seemingly leaves no damage to popstar. It actually leaves pop star better off than it was before the "explosion". So it's like... the opposite of an explosion.
So, there is no evidence Kirby has the durability to survive Majin Buu's attacks, or the strength to hurt Majin Buu. The only way we've seen Majin Buu hurt beyond his recovery abilities was by Goku's spirit bomb. This is an attack that it made of a unique kind of energy, and is of a strength that Buu cannot create. If Buu can't create it, Kirby can't use it against him. Meanwhile, Kirby can be hurt by things like getting bonked with a hammer, walking into spikes, or bumping into a Waddle Dee. Pretty open shut case.
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