“Standing with poor posture” means standing in a way that is not correct or proper, often hunched over, slouching, or with misaligned body parts. Some synonyms for this could be “standing in an ungainly manner” or “standing with improper stance”.
“Standing in an ungainly manner” emphasizes the awkwardness and lack of grace associated with poor posture. For example, “He was standing in an ungainly manner, with his shoulders hunched and his back curved.” This shows that the person is not standing in an attractive or correct way.
“Standing with improper stance” focuses on the incorrect positioning of the body. “She was standing with improper stance, her feet turned in and her head drooping.” Here, the term indicates that the stance is not as it should be, similar to standing with poor posture.