A credible face champion holds onto the belt by beating his opponents legitimately and with pride. A credible heel champion holds onto the belt by cheating his opponents and taking every opportunity to win. So yes, he IS a credible champion.
Now I'm gonna rant. Why do half of you guys seem to think that every champion has to be able to beat his opponents fairly? What's the point of even having heels if you just want everyone to be Cena? MOST OF YOU DONT' EVEN LIKE CENA! Your opinions seem misguided and misplaced, and you're pulling a serious line of thread from the fleece of kayfabe. You're not meant to like the Miz, and so you don't. Does that mean he's not a credible champion? No, it doesn't. You think you're smarking, when you're actually marking.
In my personal opinion, he's actually one of the best champions to hold that belt in quite a long time. Orton and Cena killed that belt, and Edge didn't help either, and now Miz is bringing back prestige to the belt by holding onto it, and actually getting a run out of it, instead of dropping it and winning it back at every single fucking PPV, a la Orton and Cena.
As for "Miz isn't credible because The Rock interferred". That's hardly Miz's fault anyway. He kicked out of the Super-AA, which hardly anybody ever does, and then escaped the match with his title. All intents and purposes, he should have left Mania the champion right then, with his head up high. It wasn't HIS fault they both got counted out, it was Cena's. Cena put him over the barricade, and then onto the concrete. I don't see you saying Cena's not a credible challenger? And as for the Rock interfering, why is that Miz's fault? Did you bitch when RVD pinned Cena after Edge had interfered too? Hell no you didn't.
Miz is as credible a champion as we're gonna get nowadays. He's shown for the last year and a half that he has more than enough to hang with the big boys. It's just a shame that we're possibly gonna get Orton vs Miz again. Miz may need some carrying in the ring, but at least he's not as bore-inducing as Orton. Oh, and unlike Orton, he can actually hang on the mic without sounding like a broken Kraftwerk record.