Diplomatic Immunity can be used both defensively and offensively. It can be valuable to select a high damage target and try to kill them.
Try using Paragon of Demacia's Movement Speed burst to position your Heroic Charge.
The target of Diplomatic Immunity can still disable and deal damage to Poppy. Meanwhile, Poppy is usually very aggressive while the spell is active. Try to punish her for selecting you as the target.
Fighting against Poppy in the jungle can be dangerous due to Heroic Charge. Try to catch her out in the open.