There are two types of humor: that about classical music from the outside, and that about it from the inside.
One of the best insider's takes on academic serialism was done by a serialist named Humphrey Searle (a neglected genius of symphonic writing, IMO). It was at the Hoffnung Festival (CDs of which are still in print today), where they'd play works like Leopold Mozart's Concerto for Hosepipe, the Grand, Grand Overture mentioned previously, and larger works like Let's Fake an Opera (or, The Tales of Hoffnung).
Searle's contribution, aside from a setting of Lochinvar, was a piece called Punkt/Contrapunkt (a takeoff on Stockhausen's Kontrapunkt). It features to German academics babbling away for nine minutes, after which the piece they've just analyzed turns out to be only thirty seconds long.
It also has the best opening line in the world: "Music began when Arnold Schoenberg invented the tone row."
From the outside, I don't think it gets any better than Dudley Moore's
setting of Little Miss Tuffet in the style of Britten as sung by Peter Pears and a "Ballad of Gangster Joe" which is a pitch perfect parody of Weill and Brecht.
Personally, I think Harrison Birtwistle's
Punch and Judy is one of the funniest things I've ever heard (but I imagine many wouldn't agree; Britten walked out of the first performance).
I was lucky enough to see Ligeti's
Le Grand Macabre in San Francisco, and I thought I'd die laughing.
