It's good for Bitcoin publicity, and it's a symbolic act. It's also nice that they're leaving it there for anyone who can get to the Moon to collect it. However, it's not cost-effective at all. It'll cost at least dozens of millions of dollars to fulfil this trip, and Bitcoin's worth only around $50k. When we still have problems here, on Earth, I don't find it wise to make trips to the Moon only for such silly purposes. Also, it'll be in physical form there, which kind of defeats the whole point of Bitcoin because it's in its nature to be digital and people should get used to it.
BitMEX don't do it without strategic marketing plan. We can think that they do a non-sense thing but this activity will catch many attention from the crowd and BitMEX will attract many new users to their exchange.
It can be more effective than any traditional marketing effort and its noise will be huge. Such events don't occur too often. A space flight to the Moon which carries a physical Bitcoin. Hey it is an excellent idea but it seems Elon made it first last year with Dogecoin.