This is a common problem that comes up when doing 2D Games. Luckily with some simple math you can fix this really quickly.

//What is the difference in position?
Vector3 diff = (lookTarget.position - transform.position);
//We use aTan2 since it handles negative numbers and division by zero errors.
float angle = Mathf.Atan2(diff.y, diff.x);
//Now we set our new rotation.
transform.rotation = Quaternion.Euler(0f, 0f, angle * Mathf.Rad2Deg);