I don't think Thunderlord's is OP but I don't get why every damage over time ability triggers it.
It's pretty abusable by anyone with a point and click DoT or a DoT with a wide radius. Like, step into a Morgana pool for half a second and it goes off. Get hit by one Cassiopeia W and it goes off. Thunderlord's is there to rewarding champion for landing their full combos or making a successful trade. It makes no sense why a champion with a DoT can sit back, hit one ability and get the same treatment without committing to any risks. Make it so that Damage over time abilities only apply one stack towards procing Thunderlord's. That should help solve the issue of everyone taking Thunderlord's and push DoT champions towards Deathfire Torch, which is intended for them to begin with .
Edit: it has been brought to my attention that not every single DoT ability triggers Thunderlord's but my point still stands. For the ones that do, it is abusable because you can bully the other laner without committing to anything or taking risks. For instance when playing Hecarim, you can proc Thunderlord's by just by standing close to the enemy laner, turn on your W and wait for 3 ticks. That's free damage for just being near the enemy.

Flamethrower... all it has to do is Q+E and you get hit by thunderlord
You'll walk by a minion, and out of nowhere you get hit with a thunderlords.
Can apply Thunderlord's in two(?) autos with E
-I think Aatrox has a similar interaction with his W but I haven't tested this
Can apply Thunderlord's with one E cast