doesnt mean they are bad, Alot of winter fuels will make your car stink, this is normal with the winter additives and fuel with higher sulfur content will also make them smell. Other things like short trips when the cats can't get hot enough to burn out the impurities can also make them smell rotten.
The only way to tell if the cat is bad other than obvious things like it rattles or fell off is to run it on a gas analyzer and see if they are working correctly.