So, could I understand it as the law of nature?

That might make the translation lose the "joke" feeling. I once tried to keep the humorous tone.

We here mention much less "God" than you do, you know. And when speaking of God, we do in full awe; never dare think the God is humorous or can joke with us. So we have no such saying as "cosmic joke", a joke played by God.

Do inform me if you see any corrections needed in my written English.