Depends on the flight. If it's short, then not a big deal. If it's a daytime flight and long I'd let the person know. Not everyone realizes that they snore or how much.

Should this be an overnight flight, then it's fair game. Some people snore when they sleep and you should expect most people to be sleeping on an overnight flight.

You should always have ear plugs with you. Quite likely, you wouldn't even have needed to start this thread.