I've been thinking about this for a long time: how could it be possible that you can acquire leathers above average quality from a rathound/pig/siphoner killed by full-auto lead storm or uncountable knife stabs? It's basically no reason.
Mechanical damage should become a threat if you want to gain high quality leather or caparace, and some other damage/status effect should be totally avoided. For game mechanic, each individual attack decreases critter component quality for a certain percentage, and leather loot shouldn't be found on burn/acid entanglement effected critter, as follow:
Damage type Quality decreased per attack (leather) Quality decreased per attack (caparace)
Blunt Mechanical 2% 15%
Thrust Mechanical (bullets) depends on caliber, 2%/5%/7%/10%/12%/15% depends on caliber, 5%/8%/10%/12%/15%/20%
Thrust Mechanical (bolts) 5% 5%
Thrust Mechanical (knives) 5% 1%
Fire (ignore burning area) 15%, remove loot if burnt 4%
Acid (ignore acid puddles) 15%, remove loot if acid entangled 4%
Corrosive Acid 25%, same as above 25%
Energy 8% 8%
Spikes and serrated knifes on armor/boots/gloves count as knife attack.
Beartrap, bio, electrical and mental damage wouldn't affect. Cold damage neither, but consider that cryogun bullet and cryokinesis are mech/cold hybrid, big, look like creepy "ice-spear", I think they are equivalent to .44 caliber firearm. Cryo-orb shards count as 7.62mm rounds.
Hence you'd receive a tattered fishing net or even nothing, if you don't hunt properly.
IMO it's a good way to improve immersion, but I don't know if there's so much script work that make it hard to achieve.