yea, thornmail needs changes. imo they should make the flat portion of the damage return scale with total hp, instead of armor.
problem with the armor scaling are nieche cases of champions being able to get 600 armor and upwards, while even most tanks and fighters get to about 300 to 400 armor with the thornmail. riot apparantly balanced the current thornmail around malphite, rammus, taric etc. basically if the item would be good on the average bruiser or tank, it would be broken on those 3.
the possible armor on them is just a gigantic difference to what everyone else reaches when he specializes in armor.
the difference in armor is much larger than the difference in health between specialized health tanks like sion or chogath, compared to other health stacking tanks and fighters.
for example, a sion or chogath will probably be able to reach 4500 health with a full build. while the average bruiser or tank who would buy thornmail would reach around 4000 health.
this difference is smaller than a leona with 300 armor compared to a rammus or malphite with 600 armor. thats why imo the scaling should be changed to health. it would still make thornmail better on certain champions than others, but the gap would by far not be as large. which means the item could be made better in general.