It is not necessary for good feng shui to have a mirror on the bathroom door, but we will give you some advice for those who like to use a mirror in that place.
The reason mirrors are sometimes recommended in feng shui for the bathroom door is because a mirror can make a wall, or a door energetically disappear, so to speak. A mirror also pushes the energy back, this is why, a mirror outside the bathroom door is considered good as the mirror pushes the energy away from entering the bathroom, thus the energy is prevented from going down the drain.
If you decide to hang a mirror on the bathroom door that is near your bedroom, than make sure to choose a small mirror. It is not best feng shui to face a big mirror as soon as you leave the bedroom.