While you're quite right about that, and that IS certainly an aspect of the Joker character as a whole, I don't really think it can accurately be applied to Ledger's Joker. When did he make jokes? When did he even make the pretense of joking? The most joking he did were some dry wisecracks and general weirdness. Challenge authority he certainly did, but I wouldn't say he even tried to use jokes to do it - he didn't even use the DCU Joker's 'life is a joke, and I'm trying to make people see that' shtick, which would have fit in perfectly with the Ledger Joker's view on life. (Mind you, it's been a while since last I saw it, so I may have forgotten a few things, but I saw it twice initially, and have read countless quotes from it and dissections of it on the internet, so I'm pretty sure I'm right about this.)