When you look at a person, what do you look at?
According to a book, Japanese people look at a person's eyes, and Westerners look at their lips/mouth.
Therefore, Japanese people don't usually wear sun glasses.
They feel that not showing their eyes would be impolite.

Unlike the Westerners, the lips are part of their expression, so they don't cover their mouth.

You'd probably see a lot of people wearing face masks in the winter.
It may look strange but it is very usual for Japanese people wear a face mask in everyday life.