Evil Scarecrow == Good use of humour.
Tenacious D == Bad use of humour
Why? I think (and this is obviously just my opinion) groups like Evil Scarecrow genuinely love metal but can also burst the bubble of pomposity that sometimes surround it.
Tenacious D on the other hand just seem to take the piss out of metal.
Ergo, humour coming from a good place is healthy. Taking the piss for the sake of it is just annoying.