alreadytaken - nope, if you're going to quote me, I'd have to say that the 'miserable but competent' types are usually under no illusions as to their own performance.
I work with one, pleasant enough lass, who will cheerfully tell you she's all about the holidays & will be quitting as soon as her dh says she can 
. She does a just about passable job, stays under the competency radar & won't be missed when she leaves.
What you actually can't do these days as a teacher is what you're suggesting - be a miserable git, & yet be 'self confident' & think yourself officially wonderful. You have regular lesson observations, which if you are a misery in the classroom will come in at a '2- good' at best, & that's assuming your exam results are impressive.
There are teachers who are grumpily, but competently, going through the motions - but they really aren't doing so in a happy haze of imagining themselves to be marvellous! 
On the whole, if you want to know why we moan so much, you can generate some reliable chuntering just by telling us we moan too much...