But that's mostly not on them, but rather on writers who don't have a sense of scale or how devastating in real life some things are that such characters often just shrug off. Since their data is based on these writings, it comes over as biased while it's just based on the math that the writers forgot to make. Best exemplified by the Kirby vs Boo fight and the giant pan thrown around the sun, which, with the time it took, would only be possible if Kirby is more powerful than all current space rockets combined. He certainly wasn't supposed to be that strong, but his feats are just explainable that way.
In Cloud's case for instance, it's based around the fact that all the spinoffs and in-game cutscenes seem to agree on him using those exact materia, and is always using the buster sword despite the games giving him much better options. A similar thing happened in the Tifa vs Yang fight with the materia. This is the problem they're facing with videogames: The often huge difference between what's canon an what's possible in-game. Comicbooks have the problem between the different Iterations of a specific character: Do you follow a specific one, or make an amalgamation of several versions of the same character?
So yeah, sorry that I had to play the devil's advocate here, but lots of the reasons the results look biased are out of their control and just different parts of the source material taken at face value, which is necessary to determine their contestants' strengths and weaknesses.
All that doesn't mean I agree with them all the time. For instance, I don't see how the Spartan could have won against the Doomguy since the Doomguy in-game can take rockets to the head and survive, something the Spartan can't do, which should make the Doomguy way more resilient than he was in their research. However, I would prefer if they remade the episode not as a duel as usual, but as a deathmatch between them, Serious Sam, B.J. Blazkowicz, Duke Nukem, Quakeguy, someone from Unreal (tournament) and Turok in an 8-way Deathmatch. Get Psyched indeed!
Nah, it's not the writer's fault, it's theirs.
To use the Kirby thing as an example. First of all, they claim that Kirby survived a planet sized explosion in Triple Deluxe. Now, Kirby is a pretty mellow series with no planets exploding. What they were referring to was actually a scene where the vines covering the planet burst into light, an event which did not do any physical damage whatsoever. There are tons of mistakes like this.
As for the example you mentioned with Kirby throwing a pan, you said it yourself. He certainly isn't meant to be that strong. And there are tons of examples in the anime series and games of him not being not nearly that strong. In fact, there are WAAAAAAAAAAAY more examples of him not being that strong than of him being that strong. Almost every other example shows that he is not that strong.
In any kind of research, you throw out outlier data, and that's what should have been done in this case. Because consistency is more important than a sight gag in one episode of a show that isn't actually canon. Death Battle does the exact opposite, and takes the most extreme example, not caring if it contradicts almost every other time the character is shown. They should be throwing out this data, And that's actually what people debating these things on the internet typically do. If something is bizarrely out of range for a character (for example Spider-man beating the Hulk with ease or Venom smacking around Superman which have both happened) you ignore it.