I was interested that the action is exclusively set in Africa. There is mention of Europeans being in Cairo, and this making it a cosmopolitan city, but I don't think we meet any of them. I was also interested that the astronomer character, whose delusion is that he is in sole control of the planets and the weather, is aware of the moons of Jupiter. We are clearly meant to read the African characters as disaffected young English men and women, and that is how they are portrayed (with a touch of Orientalism) in the illustrated editions on-line; I don't think Johnson is really trying to say anything about Africa (though he had translated Jerónimo Lobo's book about Abyssinia twenty-five years earlier).
It's striking that this was written 250 years ago this month, the same year (1759) as Candide, which has a similar basic concept, but the timings I think are such that neither Johnson nor Voltaire can have much influenced the other. It seems to have been the last fiction (indeed, the only prose fiction) that Johnson published. It is somewhat pessimistic but very engaging.