The context of this quote is as follows:
He halted and, with bewildered and horrified eyes, stared round him at the khaki mob, in the midst of which, overtopping it by a full head, he stood. “How many goodly creatures are there here!” The singing words mocked him derisively. “How beauteous mankind is! O brave new world."
John the Savage has come to visit his dying mother, Linda, who he loves very much even though she was an inadequate parent to him. She is in the Park Lane Hospital for the Dying. As he arrives, he confronts two sets of identical twin Delta workers. One set has rounded freckled faces and red hair, the other set sharp beaked noses. They all ask him angrily why is "pushing" to get through them. They are the "khaki mob" he sees.
John is not used to mass hatchings of identical twins, which is why he is "bewildered" and "horrified." He is also focused on his grief for his mother, so is startled to be confronted by this hostile mass of strange-looking people.
The words from The Tempest mock him because in The Tempest, they are sincere: Miranda, who has only known her father, is genuinely delighted to meet the other men who shipwreck on her island. They are truly "beautous" to her. However, this mass of men, in this "brave new world," are hideous to John, a degradation of the human race. From what Linda had told him all his life, he thought the people in the World State would be wonderful, but he has to face the bitter truth that they are not.