Thornmail isn't under powered and here's why

No. Thornmail is a toxic item in my experience and you need 2 lifesteal items to counter it which isnt always possible in some cases for example say your playing sivir or lucian your build looks something like this most games are pretty much needed leaving you with 2 item slots left so you can build 2 life steal items right? Not really because with 2 items if the enemy has thornmail odds are your gonna need a / as well leaving you one item slot for life steal and most tanks have some kind of hard cc so your most likely going to go over leaving you with a whopping 10 percent life steal. some other champions can build more such as vayne or kalista or yi. but for half of the adcs right now the build i just mentioned is the most likely so lets do some maths here thornmail does

UNIQUE Passive: Upon being hit by a basic attack, reflects magic damage back to the attacker equal to 25% of your bonus Armor plus 15% of the incoming damage.

This is calculated before armor so an adc with the build i mentioned above has roughly 343 attack damge (before runes and masteries) so per auto assuming you crit the damage reflected is 129 dmg now after life steal it does 89 damage but since life steal is calculated after armor reduction you do less so lets do more math here by adding armor into the equation so im going to use mundos stats (because mundo is just the most classic exaple of a tank) to calculate the rest of the damage so the most popular build for mundo is and giving him 200 bonus armor (before runes and masteries) so thats an extra 30 damage dealt per auto attack so by these maths thornmail does 129 damage per auto (of course more if its built with more armor) meaning that the adc will kill themselves taking 65 so 25 percent reduction 97 damage per auto attack it would take 20 auto attacks for an adc to kill themselves that a lot but thats assuming the dont take any other damage.

Also it gives you 100 armor thats a lot of armor