Well that's one way to look at it, I guess.

But I think of it like this:

When Poppy is full hp, those attacks that go over 50% of her current HP are hard to come by. Having armor at this point is fully beneficial.

When Poppy's HP is 30-60%, having no armor would kick in the passive for SOME skills. At this point, having armor is crippling the passive, I agree. But then again Poppy would make a lousy tank if you'd rely only on her passive mid game. However, Poppy is resilient to nuking at this point.

When Poppy's HP is 0-30%, having armor becomes a bonus. I'm not a theorycrafter, but as far as I know, Poppy's passive kicks in AFTER the damage reduction from armor and stuff.
This means that when I'm on 300 HP, and I get hit by a 150 damage AA, the armor(62% reduction) reduces it down to 57. At which point the passive says: WAIT A SECOND!!, and kicks in. Cutting the 57 physical damage down to a mere 28.5 damage!

Pretty damn good, in my opinion.
So. In conclusion, no. Stacking armor isn't a bad idea.

I'm curious as to why your building her as a tank. I'm not saying I know whether or not she was intended a tank, but now it seems her skill set isn't based around being a tank. I think if you build resistance with poppy it negatively affects her passive. As you already know her passive reduces any damage she takes over 10% of her health by 50%. If you stack resistances then your passive won't effect the damage you take. Not trying to troll man, I have the uttermost respect for poppy players. If you can respond please do! :)

Yes. Thank you for your feedback, but I did say in the Summary that this is still a work in progress. I will have 3 to 4 times more information when this is finished.

Right now I'm too busy to finish this all in one go, but eventually this will be a guide to match the Mobafire veterans' guides. This being my first doesn't exactly speed things up.

One of the reasons I published this, is that I wanted to SEE how it looks(yes, now I know that I didn't need to publish it, but I'm not going to take it down - comments like yours will help me form this into a nice build)

Thanks for the feedback, and I'll definitely invite you back here when this is finished :)

There are quite a few things wrong with your guide at the moment. For one, I would switch one or both points from Defensive Mastery to Harden Skin , all minions and neutral monsters deal physical damage, and combined with damage from champions Harden Skin is a much more viable choice. Another issue would be your Sheen that is left to be built or replaced by nothing, you have no item section so it is rather confusing to see what to take especially because you have an Abyssal Mask at the end boosting your ability power. The third big thing about your guide which I would change is maxing Paragon of Demacia, especially if you are building the tank version of Poppy, this spell is an armor buff which is very needed if you are going to be tanking, much more needed then the bonus damage you get from Heroic Charge, and when you level up Heroic Charge the stun doesn't even increase in duration, making it fairly useless for any non ability power based Poppy to max it before the other spells. The last thing is that your guide is very short and doesn't explain why you chose what you did, in fact, my comment has just about as much text as your entire guide does, explain what you did and why you did it, and try to add some coding into your guide, there are guides on MobaFire that will tell you how to build a guide and how to make it look great. You also should add and break up some sections, maybe add some screen shots to prove that tank Poppy works as well as you claim, you could also add other items that you find viable, like if you are going for magic resist, a Force of Nature works amazing on Poppy because the more movement speed she has, the easier she can chase down and finish opposing champions. It's a decent start but please incorporate some of these changes into your guide, then message me and I will reconsider my negative vote.