2D Physics Rotate Sprite Level With Floor

You shouldn't need to be worrying about the z at all--in transformation terms. I'm not entirely sure what you're getting at--but I think I do. If your sprite is a quad (a flat square) and you are rotating it with physics all the time and need to 'reset' it to be flat with the ground couldn't you just set the rotation(I think the z axis) back to zero?

Here's a quick easy fix.

 //this goes before the Update() loop because you don't want it changing every update
 private Quaternion zeroRotation = whateverGameObject.transform.rotation;
 
 //when it's time to make the sprite 'level' with the ground again just set the rotation back to this
 
 whateverGameObject.transform.rotation = zeroRotation;

Dealing with rotations can be difficult--when I can I try to use a placeholder Quaternion variable rather than figure out the very complicated math behind it.

*EDIT: I think now--you mean you want the physics to auto rotate the sprite back to 'zero' --not just change it immediately to zero -- ie. a smooth turn to zero. You can use the same placeholder quaternion for your zero position and rotate the sprite along the axis in question(I'm assuming z) and check if it's at the zero position or not--if it is then stop rotating.

 Update() {
 //check if the transform is at the zero rotation
 if(transform.rotation != zeroPosition)
    // Slowly rotate the object around its z axis at 1 degree/second.
    transform.Rotate(Vector3.up * Time.deltaTime);
 
 }

I will say that if physics are involved--your quaternion may not be an integer--so you would potentially never be equal to the zeroPosition quaternion. You can look into doing a looser comparison with some buffer room ie. 1 degree of wiggle room--but this is a quick answer. Note that this will always attempt to rotate the sprite back to zero indefinitely--so you may want to set a trigger for it specifically--I don't know all the details.