He's the centerpiece of the show. Therefore, it's all Punk's fault the ratings are horrible.
Cena seems to become the new Orton. He has some star power (a lot more than Orton and ten times more than anyone else) but is relegated into having meaningless match on the shows, which kills his drawing power. I'm pretty sure he'll help the ratings in a couple of weeks if he takes the title and ends the biggest reign of terror in history!
To be fair, you could argue that almost all of the matches in WWE these days seem pointless. Like, how many times do we have to see ADR vs Sheamus, Sheamus vs Ziggler and the like? The more a match takes place, the more the later matches feel like "wait, why do I care?"
Though with MNF starting I don't think a Cena title win will do much.
And can you honestly say Punk's reign is the biggest reign of terror in history? Really? Come on, man, there have been worse. Its been a very empty reign and the title has been very devalued, and I understand that you're not a Punk fan (which is fair, to each his own), but can you seriously say that David Arquette's reign, for example, was better than Punk's? Keep in mind, you didn't say which title reign, just that it was the biggest Heavyweight title reign of terror, which to me means it could be the WCW title, WWE title, TNA title, etc. If you mean WWE title...well its certainly up there...or down there, whichever makes more sense. Miz's reign was pretty bad, and he main evented a Wrestlemania. Rey had the belt for 90 minutes...Hogan's 1993 reign he didn't defend the title once (even though Raw was on the air) until KOTR, where he lost.