How can you write on the bathroom mirror?

A bathroom mirror should be at about eye level, and certainly not above it. Ideally, the bathroom mirror is above the sink. When in doubt, gather a group representative of those who would be using...

Temperature deviation among the steam and surface temperature on the bathroom mirrow causing condensation.

Fog appears on bathroom mirrors like how your hot breath appears on glass when you breathe on it. The hot vapor in the air mixes with cool glass and fog appears.

It depends what the intended use is. An 'ordinary' mirror - is flat, while a shaving mirror is usually convex - making the image seem larger.

Could be condensation