nope. The 15% damage return is before mitigation, meaning if you get hit for 1000 damage it will return 150+25% of your current armor.
Lifesteal on the other hand is based on how much damage is dealt after mitigation and then calculates the healing return off of that amount.
What does that mean without crazy complex math? you are hit by an auto attack that crit to deal 1000 damage before mitigation but deals 400 damage after mitigation.
Thornmails effect activates and returns 15% of the damage of the attack before mitigation (aka 150) in addition to dealing an extra 25% of however much armor 60% physical reduction is (lets just round it to 50 even though it is probably more). total damage returned? 200
The attackers lifesteal kicks in and heals them for 20% of 400 damage, which is 80 health.
thornmail hits for 200 damage and they get healed for 80 health = they are still taking 120 damage without factoring in magic resistance (closer to 95 if you did)
lets take the same situation with old thornmail and its 30% return of damage taken. attacker still has 20% lifesteal and hits you for 1000 damage reduced to 400 after mitigation.
Old thornmail did not look at the damage before mitigation and instead calculated off damage after mitigation and so returns 120 damage
The attacker still returns 80 health from the 400 damage they deal.
old thornmail hits for 120 and they heal 80 from lifesteal = they only take 40 damage (closer to 30 after resistances)
so you tell me, would you rather have thornmail return 120/95 damage or 40/30 damage
######by the way 25% of 400 is 100, not 60. Also armor penetration like last whisper and yomuus does not effect your damage calculated from armor, Debuffs however do so BC would reduce that return. Bork does not deal %max hp damage either, you need to read the items more closely.