Hot messes' are appealing for a variety of reasons, most notably because they're generally unexpected, capricious, & agonizingly provocative. Additionally, numerous contingent factors make duplication rare and continual repetition virtually impossible.
No one set of guidelines can perpetually determine what distinguishes a "hot mess" from an above-average train wreck. Regardless of the circumstances, you know it when you see it; because they are typically conspicuous, and obviously they are always awesome.
**The term "hot mess" can also characterize a unique setting or situation, although in this context the usage parameters become more difficult to quantify.
Eric: "Yea, she came in this morning to find the rest of her clothes. She looked terrible, and awesome."
Justin: "Damn, hot messes' like that don't come around very often."
"Girl came in to school this morning looking a hot mess, her hair all jacked up and slouchin' in house shoes."
You: You know, I love her to death but she's such a hot mess when she drinks
Friend: I just can't get that chick out of my head. I mean, I know she's a total hot mess, but I dig it.