Actually, the correct definition is “a Zombie left out in the sun during August.”
It may have originated as post battle clean up, unexploded ordinance strewn about.
I believe the connotation is “not just messed up through circumstance, but severely messed up through some aspect of craziness or instability on the part of the person”.
Like if your home was destroyed in a hurricane and now your life is all messed up, that’s just a mess. If you have a heroin habit and the syphilis you caught from prostitutes was left untreated and now you’re going insane – that’s a hot mess.
I work with an older black woman who uses it frequently, and she uses it to mean a “big, tangled problem that will take awhile to straighten out.”
In this context, ‘hot’ is not literally hot, nor attractive, but more like ‘extra’.
The object - person/house/whatever isn’t just a mess, it’s more than a mess. It’s a hot mess.
In my experience, there’s little to separate a mess and a hot mess. It’s just more descriptive - like putting ‘super’ in front of something.