Well I think it's something to do with him being a happy, bouncy (Jamaican?) guy, unless they changed his gimmick completely he couldn't be taken seriously because atm it wouldn't look out of place on a comedy character.
I don't know why you think he couldn't be taken seriously, but what does it really matter? Like I said, Rey Mysterio couldn't be taken seriously, but because he was over huge, they gave him the title. Hell, I could even argue that John Cena can't be taken seriously. Am I honestly supposed to sincerely believe that John Cena can beat The Great Khali? I hope not, but it doesn't matter because the crowds don't care.
The Miz can't be taken seriously, but that didn't stop him from being WWE Champion, did it? Now, I suppose your defense is gonna be that The Miz isn't supposed to be taken seriously to begin with as he's a cowardly heel, whereas Kofi would be expected to be taken seriously as a babyface, but really, whatever your opinion is on why he can't be taken seriously, it has no business basis to it. He's over huge everywhere he goes. The crowds clearly take him seriously because he gets huge reactions, and not in the way a comedy character like Santino gets huge reactions. Besides, he's already beaten most of the people in the company anyway, so what's the concern?