My apologies. I just listened again to what MTT says before the Prom perfomance.
What he says is different. He says conductors have struggled to make sense of the finale whereas for him the finale doesn't make sense. What mahler does is a lot of jump-cutting, from one type of music to another. Finally that mahler may have found the material both amusing and terrifying.
I like humour generally, and certainly welcome it in art music. Plenty of examples spring to mind; any amount of Haydn, like the end of the Farewell Symphony, Beethoven, Rossini or course, like the Il Bruschino overture; Ivert's Divertissement. Comic operas, for goodness sake
I like you're idea that Mahler was laughing at himself. That's very apealing. All , no, most Cancerians (e.g.Mahler) have an off-the-wall sense of humour. And there are any number of Jewish comedians.
The finale begins as tho' we're clearly going to have fun now, and there's something comic, too, about that decrescendo on the penultimate chord. At one performance of the 7th, when the trumpeter plaed his gentle trill about 5 minutes from the end, I noticed the odd titter - it sounds a bit comic, and I like to think Mahler would have approved
Ivor