The fact that most people accept Miz as a credible champion is precisely the reason that the championship itself is still lacking in credibility. I do not care if heels cheat to win. However, WWE overuses the fluke runs to a point it ceases to make sense. Flukes are supposed to be rare. The way WWE does it anyone should think they could suddenly get a bad attitude and hold the title for at least 3 months through pure chance. That isn't something that builds credibility. Miz is still no better than average at basically everything and no better than good (see: not great) on the mic. The length of time he holds the title does not change any of this. Marginally better than the titles low point in value? I might buy that but marginally better than a low point is still quite bad, especially when that low point probably occurred during his actual reign.
So, because YOU find his skills to be average, that lowers the credibility of the belt? Wow, behold the power of Shattered Dreams! I guess by your logic that anyone who YOU deem worth would bring prestige to said title. Care to give us a name oh mighty one?
All kidding aside, what you said is ridiculous, every single part of it. By no means was Miz's win a fluke. The guy was pushed at a similar speed to guys like Austin, Bret Hart, and Rock and two of those men could be argued to have won their first title in a way more fluky fashion. Hart won his first belt a few months after dropping the IC belt (like Miz with the U.S.) but did so on a house show! Spur of the moment booking? As for Rock, he was an Intercontinental champion a few times before even sniffing the main event and he got his first title in a swerve that featured the McMahons helping him beat Mick Foley despite Foley not tapping out (it was one year after Hart/Michaels and somehow this was deemed a good idea). Now, lets look at Miz.
After his split from Morrison, Miz challenges Cena. At this point, he was NOWHERE near Cena's level and it resulted in him being embarassed. In this time though, Miz cut some great promos and people started to take notice. Thus, after he got fired and rehired in storyline, he came back with a much angrier attitude. He wanted to show people he belonged and he did so by winning the United States Championship. Winning a midcard title certain helps build a guy if he holds it with honor and Miz did. Someone will argue that his defenses weren't as frequent as they like and I'd agree, except that he also became Tag Team Champion concurrently and that was defended. It also was made mention by Miz basically every chance he got and the fact that he was damn proud of his belt brings it more credibility than any defense against R-Truth could.
This was a kid getting TV time every week and it was pretty clear that he was going to get to the main event. The way it was done was Money in the Bank. Like Edge, RVD, and Punk before him, Miz won this title, but he did so against actual main eventers. To win that case, he had to beat guys like Jericho, Edge, and Orton. I know it's not a typical match, but that's still no easy feat. I'd say that's building him pretty well, no?
Moving forward, he loses the US title to Daniel Bryan so he can fully move up. Yes, he tapped out to Bryan. No, it doesn't mean he loses all credibility. Actually, his cash-in probably gave him more credibility than any match against Bryan could. He cashed in on a weakened Orton but it became a match rather than a squash. Miz actually had to work to get the win and it looked like he would be the first MITB winner to lose his opportunity (something quite a few people predicted). He wasn't, and he ended up winning that match after hitting his finisher. Quite a way to start a title reign I should say. You can call Swagger a fluke way moreso than Miz and he didn't last. Miz is already light years ahead of Swagger.
So now Miz is champion. Orton is gunning for him but he can't beat Miz. It doesn't matter HOW Miz keeps retaining, the fact of the matter is that he does. Ask Ric Flair if he cared when the Horsemen helped him retain. I'm sure he appreciated the help (well, either them or his forearm that was fond of striking genitalia). He has been the perfect heel champion, making you BEG for someone take the belt off of him but yet finding ways to win at every turn. On top of that, he's quick to brag about it every time he does, getting you to further hate him.
Now for the capper. The guy he challenged 18 months prior is now someone he is almost on a level with. He isn't there (because he shouldn't be as he's supposed to be the underdog that's expected to lose but finds a way to win), but he's at a point where he feels his savvyness can best Cena. Now, they have a legit match at Wrestlemania instead of a squash. In that match, Miz kicked out of the ATTITUDE FREAKIN ADJUSTMENT! I can probably count on one hand how many people have done that. If that doesn't legitimize him, nothing will. Fact is, he's been legit for a while anyway, but that just solidified it. Even if he lost, he would have been a made man, but he didn't lose! He retained his belt on a double countout and then he won with Rock's help. Still, the company trusted him to walk out of Wrestlemania as a heel champion. That hasn't happened very often. The others off the top of my head to walk out of Wrestlemania as heel champions? Triple H, Stone Cold, and Randy Orton. Not bad company I think.
The fact of the matter is that you are basing his lack of credibility on your personal feelings for him. You feel his mic skills are good and everything else is average yet he is a long reigning champion that the company trusts. I'm sure there were people like you who hated Stone Cold, The Rock, Triple H, Kurt Angle, Edge, Cena, Randy Orton, etc. for whatever reasons. They all became stars and so too is Miz now. He is here despite your "expert" opinion on his skills. Not only that, his title reign has been one of the best heel title reigns in years. In it, he's beaten now the top 3 faces on Raw. That's pretty impressive. Not only that, he's defended it on average twice a month. To defend it on Raw a few times like he has and to defend it on PPV and STILL not lose is impressive. The kid is legit and probably the first heel since Orton to have true staying power in the main event. Like him or hate him (go ahead and hate him since it's his job to make you do so), he's become a main event heel and has certainly brought notice to the WWE title. His reign has been the best one in a long time. I challenge you to show me a better title reign than Miz's in the last few years.