Poppy's damage doesn't need to be decreased her tankiness does...

For reference:

Iron Ambassador (Passive) bonus range lowered to 300 from 400

Hammer Shock (Q) damage reduced to 35/55/75/95/115 from 40/65/90/115/140
Heroic Charge (E) range reduced to 425 from 525

(http://www.surrenderat20.net/2016/03/323-pbe-update.html)

First off, Poppy's problem isn't so much damage as she becomes super tanky thanks to a low cooldown on her passive and can repeatedly stun on low cooldown. Her damage is high, but with her usually bruiser buildpath it seems about right. But she has the Tahm Kench problem of if I target her I can't kill her, but if I ignore her she kills me.

Also the proposed changes kind of destroy Damage and Assassin Poppy (which is my preferred way of playing her) builds.

A better way to change her would be too increase the cooldown on her passive lategame and lower the shield amount . In addition slightly reduce the bonus armor and MR on her W and increase the cooldown on her E. Basically instead of taking away her damage which already has a bunch of conditions on getting full effect on it, you lower her ability to just dive into the center of a team willy nilly, find a target to stun, and then survive because of her tankiness. Lowering her tankiness means Poppy can't be as Rambo about using her E and puts more importance on finding the right time.