He means he has awful luck.
What Does Romeo Mean When He Says " I Am Fortune's Fool" When He Kills Tybalt? (Romeo And Juliet)
I think he meant that his fortune, was being with Juliet and he is referring to himself as a fool because he killed Tybalt and therefore, he is banned from Verona. His fortune is being with lovely Juliet and fool is for killing Tybalt.
That mean he was just fool by fortune. He was great because of fortune but when he became great , the fortune drag him down.