Basically an insult calling you and your father ugly.
-(Explanation)-
You and your father look alike, and are genetically related, so (typically), if someone were to say “You ugly, you yo daddy son.” They Are calling you ugly initially, then linking that same adjective to your father.
Friend 1.yo Mike
Friend 2.yeah Fred
Friend 1.i'm yo daddy Freind 2.wait why
Friend 1.i kinda got yo mom pregnant so we're getting married
Friend 2.well dad... Build me a robot version of yo mom