Rap/Happy go lucky gimmick, his fanbase, etc.
While I disagree with the title, I particularly don't see how he dresses like a ten year old. How should an adult dress then?
And the guy makes a fucking ridiculous amount of money for WWE through this fanbase. If he spoke like fucking Socrates then the younger audience wouldn't know WTF he was saying. If he did hours of chain wrestling without using recognisable chains (ie. 5KShuffle into AA into STF) then the younger fans wouldn't be able to follow the matches as easily.
The fact is, John Cena isn't aimed at YOU in particular (taking a wild guess and saying you're 16 or over). Guys like Orton, Jericho, Punk etc are aimed at older fans but Cena is there to pull money from the younger fans. Considering he isn't aimed at you, complaining about Cena is the same as watching Barney The Dinosaur and saying there's not enough sex or violence. If it's not aimed at your demographic, then you basically answer your own complaints.