Yes, its true that if you look in the mirror in dream, it can scare you. It is so because in dream we see very unusual things such as very long hair, freaky body with very long or very short hands or legs. In dreams, when we see in mirror it mostly happens that the reflection of the mirror is faded and distorted or with very bright light, which looks very scary. Its also depends on what we expect in the dream. If we think that mirrors in dreams are scary then definitely it will scare you.