The Jim Carrey stuff from the 90's was lame. The kids in the audience probably don't know who Ace Ventura is, so they probably got a kick out of thinking Cena was being a comedy genius.
Plus as already discussed, people were behind Cena because they were so against Johnny Ace, especially after the Big Show firing stuff.
I'd love to hear someone like CM Punk, Jericho or even The Rock rip Cena a new one about that promo. That comedy skit just makes me want to watch anyone BUT Cena.
I, too, thought it was awful. I hate the corny Cena trying to be funny. I love the serious Cena after JL slapped him in the face, that's the only Cena I care for (the rapping one is cool too). He just make me ashame to watch the show when he tries to be funny, like you want to hide yourself under the carpet or hide your eyes with your hands because you can't believe how embarrassing it is.
I think the only reason why Cena was getting cheered almost by everyone is because he's feuding with JL and because of the make a wish package they showed earlier.
Cena didn't really change or become more comic or more this and less that, he's just feuding with a true heel.