A person, generally female, who refers to her/himself as
a Hot Mess, but in reality is not a Hot Mess because they are either not currently a mess or, more likely, they are simply
not hot. Typically, the subject is under the mistaken impression they are hot.
(overweight) Girl: I'm
super-busy, I've lost my keys, my phone is dead, and I've got a million things to do - I'm a
hot mess!
You (to friend, whispering): She's
not hot...so really she's a Not Mess.