General:
* Disorder
* Chaos
* Confusion
* Clutter
* Jumble
* Tangled mess
* Muddle
* Scramble
* Hodgepodge
* Fiasco (for a situation)
For a physical space:
* Litter
* Disarray
* Squalor
* Untidiness
For a situation:
* Problem
* Disaster
* Predicament
* Quandary
For a person's appearance:
* Unkempt
* Disheveled
For a person's behavior:
* Sloppy
* Careless
* Incompetent
The best synonym to use will depend on the specific meaning of "mess" in your sentence.