“If you’re gonna be two-faced at least make one of them pretty.”

This is a cute phrase that points out that while many people are two-faced, there are some that have ugly faces for both. Many times two-faced people are nice to your face but then go around behind your back being ugly. What Monroe is saying here is that she’s run into people that are ugly to your face as well as behind your back. She may also be admitting that she’s a bit two-faced as well, since one of her faces was so pretty. It could have been her way of revealing that she might have had an ugly side as well. Only Marilyn knows for sure.