Poppy's passive

Valiant Fighter - All physical and magic damage dealt to Poppy that exceeds 10% of her current health is reduced by 50%. Valiant Fighter does not reduce damage from structures.

Does this passive apply before or after your resistances? So for example, if an attack would have dealt over 10% of my health but my armor/MR reduced it, would that mean the damage is reduced? If it is, why would building armor and MR be any less counterproductive than building health?

Just wondering if I should go full lifesteal instead of one or two armor/MR items.