He's not. LIke Chris-Chan, everyone thinks Karl Pilkington is a character until they dig too deep and go mad from the revelqtions they unearth, like an an archaeologist in Lovecraft.

At first I didn't know the act, and it was Gervais' podcast, so I assume they hauled out the weird sound engineer as a stunt for a few episodes.
Later, I assumed that he was a persona for a comedian whom Gervais found who had no career record, so the persona was perfect deep cover.
Having seen most of his stuff, I think he's not a pure act, and I suspect that he's being a version of himself that he knows is funny. Whether he knows why or not is anyone's guess.

I believe.

First three XFM series - real
Fourth series - starting to become self-aware
Everything else since - 50/50 split between himself as a person and as a media personality.

Yeah. If you listen to the first XFM stuff, Karl comes off as pretty shockingly normal. Perhaps not capital N Normal, but if your first exposure to him is modern Karl, the first XFM stuff blows you away. He's a person you wouldn't question the ability to operate in society of. Whether he's now playing up his idiotic side or Gervais has broken down his socialization is anyone's guess.

Yeah, that really settles any debate. Old stories are retold in an exaggerated version, and old questions Karl had about gays, dinosaurs and whatever which were previously answered, are brought up again as if for the first time, only with exaggerated comedy.

And that's fine. Regular Karl and acting Karl are both funny, and even acting Karl mines a lot of humor from his sincere reactions. He's not much different from any other comedian in that regard.