Quaternion.Lerp shaky while in coroutine?

This is very bizarre how it is shaking so much even when smoothing it out. Any suggestions on the code below so it isn't stuttering and shakey?

 public void Flip()
 {
   foreach(Transform child in gameManager.playerCoreChildren)
   {
     StartCoroutine(Helper.LerpRotate(child.rotation,
                                     Quaternion.Euler(new Vector3(0,0,0)),
                                     1f    ,
                                     child));
   }
 }
 
 public static IEnumerator LerpRotate(Quaternion start, Quaternion end, float lerpLength, Transform transF)
 {
   float t = 0;
   while (t <= 1.0f) {
     t += Time.deltaTime / lerpLength;
     transF.rotation = Quaternion.Lerp (start, end, Mathf.SmoothStep (0.0f, 1.0f,Mathf.SmoothStep (0.0f, 1.0f, t)));
     yield return null;
   }
 }