I wonder how many people laugh at inappropriate comments here and then vote them down.

I usually do exactly that (although in this case someone else got to the parent comment before me). The immediate laugh at the cheap joke is half-involuntary, but it takes a significant effort to maintain the quality of the discussions on HN in the long-term.

How does one downvote something? Is there a karma level a user should be at to be able to downvote?

Yes, the minimum is at 200pts currently