Sheamus is a bonafide main eventer. There's no doubting that. Even if he hasn't beaten anybody 100% clean thus far, he's still holding the title. He's still successfully defending the title against the biggest faces in the company, albeit cheaply. The only thing I wish they would do with Sheamus is show more of what we saw in his feud with Triple H. That was the ideal Sheamus that I want to see. The ruthless, badass Sheamus that will end your career if you underestimate him.
While what we've been seeing now, is John Cena and Randy Orton smirking their faces off at him, not taking him seriously, "knowing" they are superior to him in every way, and brutalizing him post match during title matches just to prove how much better they are than him. And he follows suit, he runs away, he has fear etched in his eyes constantly, and he can't put either of them away like he could Triple H. It's making him look really bad as a competitor, let alone horrible as a champion.
However, that doesn't change the fact that he is still champion, and he is still retaining that belt one way or another against the big dogs. But he needs to get a clean win over a main eventer as a champion in order to help establish his reign. You don't become a main eventer by a fluke. You don't remain champion by lucky wins. You gotta prove yourself in the arena of combat in order to legitimize yourself.
Everything I have to say on this matter is summed up in this one post made by Knox Beard.