I agree, it was a disappointing match with an even worse finish. But I think that was an intentional part of Cena's overarching strategy as a heel. Since his heel turn, Cena has said that he isn't going to give the fans what they want. Many fans have been asking for him to turn heel for years, but now that he finally is one, he's trying to do it in a way that doesn't please them. He hasn't gotten a new entrance song, and he hasn't changed his wrestling attire. When he comes out to the ring now, there's no flash in his entrance, and he barely shows any emotion. He doesn't want to be the cool heel, and he is no longer here for anyone's entertainment.
Cena can give fans a good match if he wants, but he doesn't want to do that anymore because the heel version of Cena doesn't want to please the fans. He deliberately worked at a snail's pace last night, seemingly trying as much as possible to not have the match be exciting at any point. I was surprised he did the five-knuckle shuffle, knowing it's his most popular move. But even when he set it up with his usual shoulder tackles, he took a lot of time to do them so as to not raise the excitement level.
As a heel, Cena has been saying he wans to "ruin wrestling," and I think the disappointing nature of WrestleMania's main event was a major part of that character goal. I'm interested to see where things go with his title run because it's unlike anything I've seen from a heel before.
I don't think Cody's stature was significantly harmed by Sunday's match. He still looked strong by kicking out of multiple finishers and not giving up to Cena's submission hold. Contrast that with Gunther, who looked like a punk tapping out to his own submission hold almost as soon as it was applied on him.