Your hairline is so ugly, it's stretching down to Bikini Bottom.
Bikini Bottom Hairline
Explanation
Explain Bear
Alright listen up, chucklehead. This joke is saying your hairline is so bad that it stretches all the way down to Bikini Bottom. You know, like the place where SpongeBob SquarePants lives? Get it? It's a long way down and it's supposed to be an insult, ya simpleton.