I do compose and make arrangements — and am firmly a mediocrity. In fact, lots of the great composers were violists: Bach, Beethoven, Mendelssohn, Britten, Dvorak, and others. Most of them also played a keyboard instrument, which is a useful tool for a composer, but a violist is perfectly placed to understand the orchestra as a whole, and is usually not saddled with too difficult a part, so they can spare some attention.

Also, violinists who composed were very common, but their works tend to display skill rather than profundity. Paganini is a good example: delightful melody, amazing technical displays, but not a lot to sink your teeth into.