I found these idioms and I believe that they transmit the same feeling of sadness. I found "have a face as long as a wet weekend" as well.
closed as off topic by tchrist, Robusto, Kristina Lopez, kiamlaluno, cornbread ninja 麵包忍者 Mar 28 at 15:39
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Without any context I would say that "face like a fiddle" can be taken more literally, maybe the writer thinks this person's face literally looks like a fiddle. I believe it is complimentary, like pretty face, although I may well be wrong.
The second phrase "as long as a fiddle" definitely means sadness, long face is quite unambiguous in English and the rest is just a simile.