I would argue that Johannesburg actually operates as more of a symbol than a metaphor. Throughout the novel, in the way that both Kumalo and Arthur Jarvis travel to Johannesburg, it seems to represent a place of evil and degradation, where people such as Absalom and Gertrude are attracted and then become associated with crime and violence. Note the way in which Kumalo is so overwhelmed when he arrives in Johannesburg:
Black people, white people, some going, some coming, so many that the tunnel is full. He goes carefully that he may not bump anybody, holding tightly on to his bag. He comes out into a great hall, and the stream goes up the steps, and here he is out in the street. The noise is immense. Cars and buses one behind the other, more than he has ever imagined.
There is a massive contrast between the country and its peace and tranquility, and the noise and crime of Johannesburg. Note the way in which one of the first things that happens to Kumalo is that he is tricked by a fellow black man who takes his money to supposedly buy him a ticket. Johannesburg operates as the place that corrupted Absalom and Gertrude and so many others like them.