Thunderlord's is far too strong on Zyra

So? She's a weaker champion (not in damage but weak as she's squishy/people don't play her hence they don't know how to play against her, etc). Let something actually work well on her for once. Jax is OP with Guinsoo's, MF can crit like 5000 damage on her ult. It's just a proc on her spells that helps her out, not an insane damage increase or whatever. Stop complaining. The more you do the more Riot might change something that a lot of people will not like.

why on zyra specifically?

I could say the same thing for other champs

As a Zyra main, I really do not see the issue with Thunderlord's on her. Maybe you're just used to Zyra in a support role where she doesn't deal as much damage. Speaking for all Zyra mains, we are tired of having our being nerfed enough already. Let her be noticed for her strength without calling for nerfs! She is designed to be a mage after all. I respect that people play her as a support but the idea for Riot to take away her ability to play as a damage focused mage is an absurd idea; people just aren't used to playing against an AP carry Zyra. She's only underplayed. Learn to play against her.