A person - more often female, but can be male, who as a result of questionable decisions - often regarding relationships with others or personal behavior - is, or has the strong potential to be, in some sort of trouble.
"That girl's drinking and partying all night while neglecting her husband and kids. She just doesn't seem to care about her work or friends either - she's a hot mess."
A person (usually female) that is in an untidy or in a messy state. Either emotionally or visually, but maintains an attractive look about her (or him?).
Dude 1: Hey dude 2, woke up with my girl last night, she was a sexy as HotMess!