Every time I build thornmail to counter an auto attack based champ it literally feels like it does nothing. Even in a situation where an enemy team is full AD if I play a tank and buy thornmail while going full armor/Regen it seems like thornmail does nothing.
if they deal 200 damage to you with a basic attack and have 20% LS, grievous wounds reduces the healing they get down to 20, with thornmail dealing 25(+10% bonus armor) magic damage back you would think they take 5(+10% bonus armor) damage, but in reality they take much less, since the damage they take is reduced by their magic resist, assuming you have 300 armor and they have 40 MR they take 39 damage from thornmail, barely more than the lifesteal heals, it would take a stupidly high number of attacks to kill them with just thornmail, and they could always just avoid attacking you. Not to mention that a chunk of the damage will be true damage if they have conquerer or infinity edge, making it likely that you die first.
It doesn't give much AP. So while you have a lot of armor, you've got no meat under that armor.
Also IE gives true damage, so ADCs care less about your armor now.
Go Amumu or Rammus.
Because Thornmail doesn't give much health (which you also need), and regen does very little into dps. It's a stat that's good against poke, but is kinda meh into DPS unless it's in insane amounts (Such as Mundo).
Also, Thornmail isn't meant to just counter AA champions. The purpose of it is to reduce sustain, especially lifesteal, not to just counter autos in general. 
is a much better item against autos, as is 
. Take one of (or both) of those two and combine it with some more health-heavy items (
or 
are all great buys) and maybe a 
or 
for more HP and to counteract the stray magic damage.
Being a tank means knowing what defensive items you need from the start. You need to adapt your build (or sometimes the tank itself) to the situation. Health is a very effective buy into Jhin and Darius, but is borderline-useless against Kai'sa (especially AP Kai'sa) due to her passive. instead will do loads against Kai'sa's damage, and will do decently into some other ADCs (primarily Corki), but is useless against most other ones. Crit ADCs will melt through regular armor in most cases, but you combine higher health ratios with , and you'll last much longer against them.
Just buying Thornmail to handle the ADC isn't a great choice. Unless you're Rammus or maybe Malphite, you typically want to hold off on it until they're actually able to get sustain.
Mostly because if its damage effect was super noticable, it would be too powerful. It helps mitigate enemy in combat sustain while focusing you, and lowering their AS, but it doesnt cut damage as much as randuins does for crit, for instance.
Because 99% of the time it is.
