Describing your main character can be a challenge. However, having your protagonist assess himself in the mirror is one of the most unoriginal ways to describe his or her appearance. If you must use the mirror, at least throw some action in. For example, “She caught a glimpse of herself in the hallway mirror and realized her blond was in desperate need of [whatever].” Another common but generally effective way, at least for hair color, is “She tossed her chestnut hair over her shoulder.” In other words, this is another example of “show, don’t tell.”
Be more creative. Note how other authors do it and when you encounter a great description, study and emulate it. It’s a challenge, but important. If you absolutely can’t do so in an original way, that’s better than not having it at all. How many books have you read where at some point you realized you had absolutely no idea what the characters looked like? I like to be told, though some authors believe the reader wants to imagine them as they will.
They’re my characters, so I prefer to convey that to the reader.