A cracked mirror can signify ones ugliness.
Broken Mirrors can be considered a recurring visual motif. Mirrors (or any similar reflective surface) can be seen symbol of vanity. Characters who find satisfaction in their image often reassure themselves by looking into one. Mirrors also provide a way for a character to reflect--both figuratively or literally. When a character deliberately breaks a mirror, it often shows what they think about themselves. Perhaps they think of themselves as ugly. Physically or mentally. Sometimes it's just a way to pound their emotional turmoil into something fragile. If the mirror is already broken, yet it's reflecting onto a scene, it could indicate that something is very, very wrong with the world.
Other Examples:Comic Books