"Hot mess" is a slang term that has two (nearly opposite) meanings. In the meaning I hear more often, and sometimes use, "hot" is just used to emphasize the "mess." So a person who just woke up in the morning and looks awful would be a mess; a person who just woke up in the morning, looks awful, and went out looking like that would be a hot mess. Miley Cyrus is a mess; Amanda Bynes is a hot mess. This meaning can also describe some things - outfits, for example - but I can't think of any good example sentences off the top of my head. In the other meaning, "hot mess" can only describe a person, and it's used to say that the person is a mess but they're still hot (attractive). It's usually used to describe women, not men. So, the really drunk girl at the club who all the men want to take home would be a hot mess, according to this definition. I hope this helps!

You don't give a context, but if you had just run a marathon, you would feel as though your hair was a hot mess. - untidy and sweaty. If you spilled a bowl of just cooked spaghetti on the floor it would be a hot mess. You actually have two descriptive words -one relating to the temperature and the other the condition of tidiness. You can have a mess without being hot, and hot without a mess. In this case the mess is also hot.

"Hot" is used as an intensifier here. While it could be literal, as Warren suggested, in the US at least, a hot mess doesn't necessarily have to have anything to do with temperature and it's often used in describing a person's appearance. Here's an article about the evolution of the phrase:http://time.com/46267/hot-mess-history-amy-schumer/