Learn English – “Went for the deep cut”

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.

From this article by Nathaniel Rich at nybooks.com (2011):

Embedded in these lines were code phrases designed to appeal to young men with certain countercultural tendencies. “Insane boys,” for instance, meant “boys who like doing drugs.” “Ben Frank’s” was an all-night diner on the Sunset Strip where musicians would hang out after the clubs had closed and wait for the drugs to wear off. “Have courage to work” meant “Have courage to abstain from drugs while we film the series.” Finally, “Must come down for interview” was not instructing applicants to appear in person; it meant “When you come for the interview, try not to be on drugs.”