The problem with Cena is that he always gets the last word. It makes it feel like his opponents don't rattle him, and robs the feud of its tension. Especially when it's against someone like The Miz, who he beats like all the time.
Here, Cena's performance was fine since it came across like Miz really did get under his skin (the swear helped). But it would have been nice if they either A) had sold Miz and Maryse having a real chance at winning or B) just having Nikki and Cena come out and fight them so that their segment is final word going into Mania.
Also, it doesn't help that everything Cena and Nikki do proves Miz and Maryse right, whereas Cena's promo tonight was just shitting on their careers with nothing to back it up. Like, Miz says Cena only keeps Nikki around because she makes him look good. Cena then...comes out and cuts a promo where he doesn't let Nikki talk but instead she just stands there and cheers for him.