This isn’t a superstition. If you look into a mirror when you’re dreaming, it’s extremely dangerous.
But why is that?
Generally, when you appear in your dreams, you are the protagonist. However, there is a good chance that you may also be in somebody else’s body. The reason you shouldn’t look in a mirror when you’re dreaming is because you might confirm this to be a reality. The moment that happens, most people go into a panic and lose sight of themselves. When you wake up, this can cause defects to your memory. In a worst case scenario, you might never wake up at all.
It’s unknown why we enter other people’s bodies when we dream. But whatever you do, it’s not recommended that you play around and try to test this theory for yourself.