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"?
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.
If you approach it purely semantically, mediocre, middle-of-the-road, average, normal, etc. seem to be absolutes that can not have comparatives or superlatives.
The same happens with absolute superlatives like
It is semantically wrong to state (although admittedly most speakers have no problem with it at all):
( http://en.wiktionary.org/wiki/perfect Its meaning is "done", "finished" originally, and as in "practice makes perfect", it indicates that it as the absolute done versus not done. Also grammatically, a perfect tense indicates something that is finished, not something that is in some state of getting finished eventually)
However, if you want to indicate that something really has nothing to distinguish it from the average, that it is actually nearly invisible, you can express that by qualifying how average it is: "his appearance was so average as to become almost invisible".
By doing this, you do actually modify the "neutral" average in a negative way (that is, I have never seen it with positive connotations.)
Mediocre, although as such neutral, already has negative connotations. You can reinforce those connotations by indicating that the mediocrity in this case actually has a stronger effect than you would normally expect of mediocrity by using
Another example might be neutrality. If a country is neutral in a conflict, that is normally an absolute, you are neutral or you are not. One could however, comment on Swiss's historical neutrality by calling them "extremely neutral".
In the same way, normally one is either pregnant or not, This is also an absolute. I have heard people describe a woman in her last weeks as being "extremely pregnant" though.
So, even though we can normally not compare absolutes like perfect, mediocre, average, pregnant or neutral, it is acceptable_ to do so in order to convey a related, but more detailed meaning of the absolute. Always be aware that the meaning will usually change:
EnglishClub has a good article on gradable and non-gradable adjectives, containing:
('Almost' etc may be used with the first two categories; freezing point can be approached on a temperature scale, but the approach to death is harder to measure accurately!)
I wouldn't include 'mediocre' in the 'extreme' category! One could easily argue that it can't sensibly be graded. However, usage often doesn't follow logic. 'Extremely mediocre' sounds a crazy juxtaposition, as SF says; 'very mediocre' doesn't sound as wacky but would probably be used to mean 'way below average' rather than 'very close to the mean'.
Even with obviously extreme adjectives, illogical-looking grading is sometimes used to save words:
'[M]ore perfect' in the US constitution is a similar ellipsis from 'more nearly perfect'.
I've had wine that was extremely mediocre. About the only thing you could note on its tasting notes was "tastes a bit like wine". There was a water-like quality to most of the palate, though not with the weakness that watery would imply.
There was nothing bad about it; no throat-scratching tannins, or bitterness. There was nothing unpleasant to the after-taste, because it didn't have the length to have any after-taste at all.
Indeed, it would almost have been better if there was something bad about it, because then at least there'd have been something of interest in the bottle.
In all, it was remarkable just how extremely mediocre it was. My father-in-law and I drank our way through quite a bit of it struggling to find something to say about it before we saw sense and opened something worth drinking.
So yeah, something can be extremely mediocre, (and quite expensive too, not that that's relevant).
The problem is, I don't think I'd have managed to convey that this wine was extremely mediocre just by calling it "extremely mediocre"; that we can find a way in which a phrase is valid doesn't necessarily make it good or useful.
'Mediocre' in the literal sense means 'in the middle', which is by definition as far from the extremes as you possibly can. In this sense 'extremely mediocre' would be a contradiction in terms.
However, if you take 'mediocre' to have a condescending meaning, which it often has, then 'extremely mediocre' would be an understatement of saying 'very very bad.' Especially in British English, this type of sarcasm works very well.
Of course you can have degrees of mediocre.
Simple mediocrity is meh.
But when someone is really mediocre, he can be described as the height of mediocrity.
And when he is so middling that he is the standard bearer, he can be the epitome of mediocrity.
And when he is the quintessential of the undistinguished, one could say
(But no one will recognize him because he has one of those forgettable faces.)
Extremely mediocre - Its adding a Degree to the Quality.
To assert a "Mediocre Quality",if we want to be more vocal and critical, it fits well - The extremely Mediocre performance of the football team....... I had wine that was extremely Mediocre...