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The short answer about viola jokes (I'm a violist, by the way) is that they come from long ago. The viola mostly plays middle harmonies, which earlier in music history usually weren't very interesting. Between that and the awkward size of the instrument, few people wanted to play the viola, and many violists were failed violinists. That's when viola jokes started. Over time, as music became more complex, composers started to feature the middle voices more, more musicians began to specialize in the viola, and the quality of viola playing increased dramatically, so the idea of violists being failed violinists is no longer true. Haydn used to complain that orchestras rarely had more than two violists who could play his viola parts. Today a reasonably good high school orchestra's viola section has no trouble at all with Haydn. But the jokes have stuck.

There might be one other reason why viola jokes have continued. While most violinists start very young (most commonly age 4-6), it's rare for violists to start younger than 8 or 9, simply because violas smaller than 12" (the size of a half-size violin) are very hard to find. In middle school orchestras, many of the violinists have already been playing for years, while the violists are either beginners or recently converted from violin. So there is usually a real skill gap in middle school when kids start hearing viola jokes, but it disappears over time.