"Some were dreadfully insulted, and quite seriously, to have held up as a model such an immoral character as A Hero of Our Time; others shrewdly noticed that the author had portrayed himself and his acquaintances…A Hero of Our Time, gentlemen, is in fact a portrait, but not of an individual; it is the aggregate of the vices of our whole generation in their fullest expression." – Lermontov
Let’s start with the source. The epigraph is written by Mikhail Lermontov, an important Russian writer, who is mostly remembered for his poetry. He was known to dabble in prose now and again, case in point: the novel called A Hero of Our Time. This work tells the story of a guy named Pechorin, a questionable "hero" who has more than his fair share of vices and flaws. We totally get why Camus would have this hero in mind – Jean-Baptiste Clamence isn’t exactly our choice for Man of The Year, either. (If you want to read more about this type of "hero" character, check out the "Character Roles.") This epigraph quotes Lermontov’s commentary on his own slightly-bad-good-guy.
Camus seems Lermontov’s sentiments, and his epigraph takes the form of advice to his readers on how to interpret The Fall. This story is not the story of one man, Camus says, but the story of every man. We all share the vices which plague Jean-Baptiste – vices like vanity, hypocrisy, and egotism. This seems to be a pretty legit summation of Camus’s opinion of his novel. On the other hand, The Fall has taught us nothing if not that we should question everything, never trust our narrator, and assume we’re being manipulated at all times. If this is true, the epigraph could be ironic. You’ll have a hard time convincing people of this interpretation, but making this argument does bring up some interesting issues.