I just think that if you were making a list of ways to make a tshirt dirty, the first item on that list would be "leave it on a shambling, diseased, rotting corpse for several days or weeks".
There's no amount of dirt, grease, grime, sweat, vomit, or shit you could possibly cake onto my clothes that would make them less desirable to wear than letting a zombie wear them for even a minute.