Even jokes aren't necessarily frowned upon, as long as you contribute something meaningful as well. A joke prefixing or postfixing a comment of substance (even if it's just a question) doesn't garner many down votes, in my experience. I think people are wary of this turning into reddit where you sometimes get chains of joke comments swamping discussion. Which is fine, because we have Reddit for those that want that, and HN for something different.