This doesn't sound quite right to me, but I can't explain why. I can understand an extreme sense of mediocrity one can get from something but does that justify the usage of "extreme" with "mediocre"?
Word Choice – Can Something Be ‘Extremely Mediocre’?
adjectivesadverbsexpressionsword-choice
Best Answer
Something can be 'very mediocre', 'completely mediocre', etc.
If it's 'extremely mediocre' though, you're crossing the line into parody. It can be used as a humorous hyperbole, the same as "fantastically bland" or "Totally averagest of them all". It makes for a very poor expression in serious contexts though.