The first and foremost thing about feng shui cures that work is that they have to be really liked by you and make sense in your overall home decor style.
If you bring a so-called powerful feng shui cure that is supposed to work but looks totally out of sorts in your space, I assure you it will not create good feng shui for you. Maybe even quite the opposite.
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, for example, it is always recommended to avoid a mirror facing the main door.
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.
You have to decide for yourself if a mirror on the bathroom door is something you would like in your home decor scheme. And if it makes sense to you.
There are many other feng shui bathroom tips for taking care of the good energy in your bathroom, explore them all before making a decision for your home.