Okay, let's say it wasn't jaden who said this. We just read it in an old English poetry book. Let's look at it as if it has a metaphor or just figuratively. Mirrors are a reflection, if our eyes aren't "real" he could mean that we are not seeing the truth, we are seeing a lie. He could mean the society concept of beauty is nothing but a false concept. This ugly person we believe we see in the mirror is not really ugly, we are seeing a lie Because our eyes aren't "real".
If you were reading this in an old English poetry book, I guarantee it would be written a lot more eloquently. If it were written as the above, the book would not have survived to be read in this modern age, people having wiped their butts clean with it long ago.
His reflection in the side mirror determined that was a lie.