you guys are all wrong, she's an amazing jangler, she's a little slow at the start but you just rush a FF and a Hydra then your clear speed will be good! Her ganks are amazing because of her ability to tower dive and survive!
Poppy can push the enemy laner to a very awkward spot. It be pushing the enemy laner to your ally so that it is an easy kill, it be to push the enemy laner away from your ally to save its life.
Meh, Poppy's ganks aren't that great at all. She has to get pretty close, the enemy has to be near to walls for her to actually do anything. Unless you have Leona top of course.
Poppy is an excellent choice for a jungler. Poppy is a good top laner but is easy to push out of lane because of her slow start. In the jungle you don't have to worry about getting bullied early game. After you get a few core items your clear time is very fast so you can farm your way into the late game. Poppy is one of the best late game champs so to use her in this way maximizes her potential. Carrying late game with poppy is not that hard.
EDIT: Poppy also has a really high farm threshold (meaning she needs a lot of farm to be effective) which she cannot hit as a jungler.
Her ganks are pretty good tho, but thats it.