I'm not a Cena fan. But it's just part of his "act" for this promo. Firstly, as Cena always says, he is coming into this match, kayfabe or not, as a fan of "The Rock". So part of his smiling is trying to look like he is marking out. The rest is just simple human psychology - if, in real life, say on the playground, one is in a feud with the proverbial "big man on campus" i.e. The Rock, the popular jock, that everyone loves etc, you KNOW you cannot win a war of words or a confrontation. So you trivialize it by laughing thus undermining them.
This is character-building ladies and gents and it is a work - it isn't The Rock coming out and cutting a promo and John Cena laughing through it because he's trying to undermine him - it's just a character building thing. It's a conscious decision by WWE creative, probably given the thumbs-up The Rock himself, to make Cena look like an infantile, arrogant douche and to make us hate him or think he has the upper-hand. And it has worked. He is trying to portray a certain character in this feud. The guy that doesn't care that he's being booed, the guy that brushes everything off - if someone strong in the real-world is being insulted and attacked verbally from all-sides and criticised, what is one natural and common reaction? Smile/laugh it off. That's what the WWE character "John Cena" is doing - laughing, smiling it off, so he comes across as the guy that's trying to brush things off but failing. That's the image we're meant to have.
It's not meant to be Cena is undermining The Rock, Cena sucks, what a terrible promo-cutter. This is PART of Cena's promo and he is doing it well. Because we were in Boston, Mas, we got a "toothfairy" chant - and a few boos for Rocky. This is why everyone is incensed about the Cena smiling. Whereas in every other promo in this feud, where Cena has been booed out the building and Rock has got a pop so loud that they can hear it in Impact Wrestling's studios, Cena has ALSO smiled through it - it's just character building!
As for Cena - heel: what struck me most about Cena's promo he cut whilst in the empty Raw arena (the serious promo), was that this is a preclude to him turning heel. He for once, took a selfish line. Usually, eg vs ADR, his promo is "you're disrespectful to these fans, to my people and to the back - I'm gonna kick your ass and just happen to win the WWE title in the process". But this was different - this was "I need this win vs The Rock, JOHN CENA needs this win vs The Rock". There was a selfish tone to it re HE wants this and HE needs this, with underlying tones of "I don't care what you fans think of me, I WANT THIS" - there was an even more underlying tone (mayhaps I'm reading too far into this) of WIN AT ALL COSTS.
And this is what I think will turn Cena heel at Mania. Rock may win; if he does I think it'll be how CM Punk won, i.e. distraction, Cena turns around and ROck Bottom. The interferer will probably be VKM. BUT, if they choose to turn Cena heel, then Cena will win unclean, with a lowblow or something while ref is down. And he will fulfil his promo from the other night of I HAVE TO WIN. I'm sure VKM will be involved in some capacity in this match, whether it's by aiding WWE's biggest star of all time in The Rock, or saving WWE's babyface from embarrassment to a Hollywood actor.