I make jokes frequently, and rarely get downvoted for them. They're rarely standalone. They're topical and non-obvious. Low-hanging fruit is punished here. Most jokes people happen to make are low-hanging fruit. It's like the reverse "lucky 10,000" effect: if you think something is funny, it's probably because it's novel to you. Meanwhile, the rest of us have seen that joke, on the internet, too many times already. Uninteresting comments go down. Also, delivery makes the joke: just like sarcasm, the medium isn't doing you any favors.