"he is an enthusiastic if mediocre painter". It looks like there's an error here
I found it in google dictionary
https://www.google.com/search?q=mediocre+meaning&oq=bionic+meaning&aqs=chrome..69i57j0l7.5047j1j7&sourceid=chrome&ie=UTF-8#dobs=mediocre
To me it looks like an error, it should be "yet" instead of "if"
What do you think?

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Greg is correct.  This construction — an “X if Y” noun — is commonly used to discuss something that has both a good point and a bad point.  Examples:  “We had a fun if rainy day at Disneyland.”   “She is an effective if demanding leader.”   This construction is usually used when the speaker wants to emphasize that the first point outweighs the second point.

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No, it's correct.
It is like saying, "He is an enthusiastic even if mediocre painter".
As well, 'if' can have the meaning of 'though'. "He is an enthusiastic though mediocre painter"

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I suppose it is not an error. It is correct. "If" there probably means "even if".