General Mess:
* As chaotic as a pigsty
* As disorganized as a pigsty
* As untidy as a pigsty
* As cluttered as a pigsty
* As filthy as a pigsty
* As unkempt as a pigsty
Specific Examples:
* As smelly as a pigsty
* As overrun as a pigsty (with rats, insects, etc.)
* As unhygienic as a pigsty
* As unpleasant as a pigsty
Humorous:
* As comfortable as a pigsty (for a pig)
You can choose the simile that best fits the context and level of formality you're aiming for.