Indeed, it tends to be the freindly, jokey comments, rather than serious ones, that get upvoted the most.

Not as far as I see. Funny gets upvoted a moderate amount, but they're overwhelmed by even the slightly upvoted serious contributions on account of their much greater frequency.

The best are comments which have serious implications presented with style - and, yes, sometimes with a touch of humor.

I don't see anything wrong with that.

Moreover, if people are self-censoring their only-decent humor but not their only-decent serious comments, that would produce the effect described.