Tattered or shredded clothing drops make perfect sense for nail boards/bats, and many other melee weapons, but not for weapons with certain ammo. Pebbles and arrows, for starters, shouldn’t leave clothing shredded or tattered. This could, possibly, be applied to weapons using small caliber rounds, also, but that needs further discussion.
Are you referring to the items of clothing that zombies drop? If so, I’m pretty sure the intent is that their clothing was horribly damaged before you came along, thanks to zombification and general wear and tear.