No, there is nothing unethical about hunting in a heavy rain. However, it's very miserable and the odds are stacked against you if you think you will shoot the big boy that day.
Deer become very skittish in heavy rain because it demishishs their senses they rely on. For a big deer, they will prefer to hunker down and not move at all and just wait out the rain. They do that normally even on a good day. It's the chance of getting something that gets them moving.