Mirror, mirror on the wall, which is the fairest of them all? A question with infinite right answers, because there is no right answer. Just your personal taste. However, there are some practical things you may want to consider. For example, a strategically well-placed mirror can create the illusion of space, making a small toilet space seem much larger and more open.
Mirrors can amplify natural and artificial lighting in toilets by reflecting light sources. This makes the space brighter and more inviting, improving visibility and the overall ambiance. Large or uniquely shaped mirrors can serve as a focal point in your overall toilet design. Or they can be used to complement the overall aesthetic of the toilet, be that modern, classic, minimalist or ornate. Or even just plain eccentric!