I don't think it should be called a "fat tax". You wouldn't say "first class tax" or "extra luggage tax". It's just that they have to pay more because they're 'carrying more'.

I don't really see a problem with it, some people weigh more than you and an extreme amount of luggage combined.

The only possible problem I could see is does it infringe on a person's sense of dignity. Like maybe next they'll be having an "ugly tax" or a "smelly fucker" tax.... "I don't want to sit next to him, he's ugly".