I was watching “Amadeus” the other day, and while seeing Tom Hulce pretending to play the piano like a mad genius, the above question occurred to me. Were there composing geniuses out there that were mere schlubs when it came to playing music that they themselves wrote?
Ravel wasn’t known for his musical proficiency at piano, and Schoenberg is also not noted to be a particularly good instrumentalist, either.
Apparently, Berlioz as well:
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(That’s in regards to hearing Beethoven music, not his own. Other cites seem to indicate that he is said not to have performed any of his own work in public on piano, either.)
I think it is going to depend on your definition of what a “schlub” is. The vast majority that I can think of off the top of my head performed/conducted professionally. Some composers, however, were considered among the finest performers/conductors of their age - e.g., people like JS Bach, Haydn, Mozart, Beethoven, Mendelssohn, Chopin, Liszt, Mahler, Richard Strauss. So, if by “schlub” you mean did any major composer not have a baseline professional competency on an instrument, I can’t think of any. If by “schlub” you mean was any major composer not also among the premier performers of their age? Sure.
ETA: Well, Berlioz was a very influential conductor, so there’s that.
And too late to edit - pulykamell, Ravel won first prize in the Paris Conservatory’s piano competition, so he wasn’t a schlub by any definition.
Oh, hmm. I coulda sworn it was him, but maybe I was confusing him with someone else.
Well, it is him I’m thinking of, but there may be a much higher bar to “mediocre” than usual:
The is also this in Google books citing him as a mediocre pianist. (“Otherwise, mediocre pianist though he may have been, Ravel was earning at least some money as an accompanist.”)
Probably not what you were thinking of as a “composer”, but Irving Berlin is undoubtedly famous. He also famously only played the piano in the key of F Sharp, and had a transposing piano to allow him to play in different keys.
Arguably, Leonard Cohen. His songs have probably gained more fame through covers than his originals ever did. In concert, his gimmick was to use a cheap-sounding keyboard. And of course his raspy voice is an acquired taste.
polykamell and Thudlow, let’s review the OP, shall we? Czarcasm asked whether any “composing geniuses” were “mere schlubs when it came to playing music they themselves wrote”. Ravel and Tchaikovsky were not schlubs. Ravel may have been “mediocre” among the extraordinary small set of pianists who get to study at the Paris Conservatory and Tchaikovsky may only have been “competent” again, among the extraordinary small set of musicians who studied at the St. Petersburg Conservatory, but they weren’t schlubs! 
Now whether Tchaikovsky was a “composing genius” or not, well, that’s more disputable! 
He’s sounds pretty good at playing the guitar in Avalanche, but I think even he admitted that was his only impressive guitar feat. And I think you’re right, like Bob Dylan, he eventually stopped playing guitar live and just sat behind the keyboard while singing, occasionally playing a few chords.
On the other hand, the subject line itself says “mediocre (at best) musicians,” so it depends on where you set that bar. I took “mere schlubs” as a bit of a humorous characterization, not literally.
So, do we have Schoenberg at least?
OK, answering this sub-question, at least as I interpret it, as to whether their are composers who couldn’t or didn’t play their own music, Ravel again comes into the conversation, although I don’t have a firm source on this:
So, if the OP is interested in that part of the question, there is that.
[hijack] Am I the only one who finds the guitar in Avalanche suspiciously similar to that in The Partisan? [/hijack]
pulykamell, it looks like Schoenberg may be the closest we have to a schlub. He took violin lessons in his youth, learned to play the cello to round out a string quartet later in life, but doesn’t look like he achieved true professional competency on any instrument. He did conduct his own pieces, though.
Ravel did produce piano rolls of some of his own piano pieces. I’m not clear on the exact degree to which that demonstrates one’s ability compared to an actual recording, but I do know for many performers of that time, it’s taken as a reasonable indication.
Strictly speaking, it’s clear that Ravel and Tchaikovsky were definitely not schlubs. But then, if we interpret the OP thus, we can more or less close the thread as I have a hard time thinking of famous composers who didn’t play at least one instrument proficently. Maybe Schoenberg, as has already been pointed out.
But if we’re allowed a bit of wiggle room, we can indeed probably include Ravel in the answer. As you said, he was among the “small set of pianists who get to study at the Paris Conservatory” but over the years I’ve also read here and there, that in that set, his playing wasn’t particularly impressive. And yet, he wrote Gaspard de la Nuit, one of the most difficult pieces in the standard repertoire (as a vengance perhaps ? :D).
It seems that Ligeti, arguably the most important post-World War Two composer wasn’t a great pianist either. And yet, as the link shows his piano études, while less than 30 years old, are already on well on their way to become standard repertoire.
In modern times, there’s contemporary classical composer John Mackey, who famously does not play any instrument. This posed problems for him when pursuing his Master’s degree in composing, since most schools’ admission procedures require an audition. I’m not sure if everyone would consider him to be a “musical genius”, but he is a two-time winner of the American Bandmasters Association’s Ostwald Award.
Just as an observation, I think the fact that we’re focusing on a very small number of composers, who flourished largely in the early 20th century, is an indication how rare it is for a composer not to be an exceptional performer as well.
More broadly, I don’t see why people are surprised that a, shall we say, less than professionally exceptional pianist such as Ravel could compose such difficult piano pieces with such gorgeous effects. Ravel perhaps wasn’t a professionally tier-1 pianist, but a composer doesn’t have to be able to actually play what he is writing to set it down on paper. Going back to Amadeus as mentioned in the the OP, the image of Salieri sitting down at the piano, tinkling away by trial and error until he gets just the right sound is almost certainly not the way that he or most composers wrote. Most composers, when they write music, just set the music down on paper as if they were writing a letter. This is still a skill that would be expected of a composer - even as an undergrad in music, the composers at my school composed away from an instrument.
I did a quick search on Ravel specifically to see what is known about his compositional process but couldn’t find anything, so I’m genuinely curious. But it would be more remarkable to me if he had to tinker at the piano before composing that if he didn’t. The vast majority of composing geniuses wouldn’t have to. The vast majority of mediocre composers wouldn’t have to. The vast majority of student composers at a tier-1 institution like the Paris Conservatory wouldn’t have to.
Most composers learn to be competent pianists as facility at the keyboard makes the compositional process much easier, although that doesn’t mean they’re concert-solo quality. And a lot of them are so-so conductors; I’ve sung for John Adams a few times and even for his own works he can’t get his head out of the score.
Performing and conducting used to be a key part of the job for composers. Now they can specialize early on (particularly as they compose on computers which can play anything back) and never have to perform a note of their own works in public.
This is true and yet, it is not unusual for composers to turn to exceptional musicians to make sure that what they’ve written is actually feasible. And in some cases, the latter even push the composers to write something even harder, which they wouldn’t have thought of. For example, both Messiaen and Dutilleux asked their wives (Yvonne Loriod and Geneviève Joy, respectively, both amazing pianists) to check whether their ideas worked out, although these composers were themselves excellent pianists.
Plus, I don’t think it’s that unusual for composers to work with a piano within reach either, precisely to try out various ideas.