Unfortunately, there’s not much we can do to make the mirror look more clear. On the iPhone 6 and 6 Plus the front facing camera is higher resolution, but anything older than that has an inferior camera that never looks particularly clear.
In addition, if you’re using a device that’s been dropped or heavily used for a year or two, dust will sometimes accumulate in the lens of the front facing camera. If you schedule an appointment at an Apple Store, they can try and blow the dust out with compressed air, but that doesn’t always work. Also, make sure that the front facing camera is not covered by a screen protectors or otherwise obstructed by dirt and grime.