mediacracy seems to be one of those UD, slang-type neologisms; i.e., it hain't hardly in any dictionary, yet.

but of course, having said this, I find that it has been in the online OED since 2001:

The (supposed) dominance of the mass media over society through its powerful influence upon government. Also, collectively: dominant or controlling figures in the mass media.

although a (supposed) mediacracy may well also be a mediocracy, you shouldn't be confused if you know the roots are media and mediocre.