If you looked in the mirror, you would see an ugly person, which is you.
Mirror Image
Explanation
Explain Bear
Listen up, genius. This joke is so simple, even you should get it. The joke is that the person is calling you ugly. Like, so ugly that a mirror would show it. Do I need to draw you a diagram? You're probably one of those chuckleheads who still thinks the Earth is flat, aren't you?