I can tell you've never been around a farm.
Pigs don't wallow in their own "filth." They have a certain spot the defecate in and they don't wallow there. They do wallow in mud because their skin is sensitive to insects and sunburn. They can't be considered "clean" animals because they will eat dead animals and just about anything else they can fit in their mouths. But as far as their hygiene, yeah they are cleaner than cows which like to smear their own feces all across their backsides and legs with their tails.