First off, check your sim's inventory for dirty items. Sometimes they'll bring food home from the restaurant and I always forget to put it away and it'll eventually go bad. So then essentially, they're carrying around dirty items which could be the cause of the disgusted moodlet. Also, if you've had any fires, or floods or earthquakes, some items may have gotten damaged that you can't visibly tell are dirty or broken. You can search each item in the room to see if any can be replaced, but an easier way to take care of it is to use the "Bless This Mess" Karma power. It should take care of the problem.