The tone of the last two lines, “was he free? was he happy? the question is absurd: had anything been wrong, we should certainly have heard,” is (in “the unknown citizen”)

The tone of the last two lines, “Was he free? Was he happy? The question is absurd: Had anything been wrong, we should certainly have heard,” is satirical.
This is probably because something really was wrong, and nobody heard anything about it. Satire is used to mock and ridicule people for their stupidity using humor, irony, or often exaggeration. This figure of speech is used quite often in literature.

The main themes of the play are: fate and free will with the inevitability of oracular predictions is a theme that often occurs in greek tragedies the conflict between the individual and the state similar to that in sophocles’ “antigone” and   people’s willingness to ignore painful truths both oedipus and jocasta clutch at unlikely details in order to avoiding facing up to the inceasingly apparent truth and sight and blindness the irony that the blind seer tiresius can actually “see” more clearly than the supposedly clear-eyed oedipus, who is in reality blind to the truth about his origins and his inadvertent crimes.