As soon as i saw the names I voted for The Miz out of instinct (and contempt for Hardy), however, after taking things under consideration, I believe there are two possible ways of looking at this,
The definition of credible is: "believable or plausible; authentic or convincing". Jeff Hardy, though a drugged up piece of shit, has held the belt for longer than The Miz and is clearly one of TNA's top stars. Meanwhile, The Miz won the WWE Championship (the greatest wrestling championship in the world) recently and is yet to defend it cleanly. He's young, not fully established, and a cheat. Every week his tactics and heelish ways make it clear he can't win matches without outside interfearence, and thus, he's not credible. Still fun, entertaining, and fucking awesome; but not as credible (in kayfabe sense.)