The story of Endymion comes from Greek mythology. There are various accounts of his story. Perhaps the most famous describes Endymion as a shepherd of such unsurpassed beauty that Selene, the goddess of the moon, fell in love with him. She begged Zeus to keep Endymion as he was, so his beauty would not fade. Some accounts suggest she loved the way that he looked when he was sleeping so much that she asked Zeus to keep him in that state. Yet another account has Endymion himself choosing to sleep forever. The end of most accounts is the same, however: Endymion sleeps eternally, beautiful and perfect.
Lestat is referring to the fact that, having preserved Endymion in all his beauty, Selene can never have him as he is permanently asleep. Her gazing on him is as futile as Louis watching Babette: he can never do anything but look at her.