'Aggressively' means very determined or forceful. "His posture was aggressively poor" means he was making a very conscious and forceful effort to achieve a poor posture. What a 'poor posture' means is a little difficult to work out here - not enough context. I would guess it means he was making an intentional effort to slouch (head bent, shoulders drooped, etc - no attempt to keep a good/proper posture.) All done intentionally as he wants people to perceive him a certain way.