As a poppy player, I agree she needs nerfs. Here are my ideas.

First, her passive gives her a massive shield that many have a hard time breaking through. Lowering its active time is a good start, but 3 seconds isn't right.

1. Lower its use time to 4 seconds instead of 3.
2. Change the shielding amount to 10/12.5/15/17.5/20 at levels 1/5/9/13/16 to make her lane phase weaker.
3. Change its damage to physical from magical.

Second, her q damage is actually pretty high for a tank and gets to a fairly low cooldown later in the game. Her q essentially becomes a sustained 7% - 14% max hp damage.

1. Lower the % damage by 1% on both ends.
2. Increase the cooldown late game by 0.5 seconds.
3. Shift more of the damage into the second proc to make it less reliable.

Next, her w is fine for the most part, but...

1. It shouldn't deal damage.

Then, her e. It's mostly fine, but some people have a hard time dealing with it.

1. Change the stun time to 1.4/1.6/1.8/1.9/2 seconds to reduce laning power.
2. Lower base damage to 30/50/70/90/110 from 50/70/90/110/130.

Finally her ult. The change that they're doing seems appropriate, but there are still some things I can say.

1. Increase the self slow to 20% from 15%.
2. change the cooldown to 150/120/90.

Note that these are just ideas for changes that I have as a poppy player that I think would be appropriate. Not all at once mind you, but maybe one or two at a time until she balances out.