not just beautiful, but mad beautiful.
mad refers to the fact that when you see something or someone so beautiful, it almost makes you mad that you didn't see them before or earlier.
mad is used to suggest that the beauty is not your ordinary beauty, but a beauty that is simply stunning and breathtaking.
ie. "yo, stop and check out that girl, she is mad beautiful!"
ie. "the dance concert was unlike anything i have ever witnessed before. It was mad beautiful; I was speechless."