The posited fame of the principle "The world sucks" is questionable; the debate over the truth of that statement predates Pangloss and Martin, and will likely never be resolved.
It is a far simpler matter to consider separately the aphorism "Never give a sucker an even break," which is commonly associated with the American vaudevillian and actor W. C. Fields. In You Can't Cheat an Honest Man (1939), Fields's character Larson E. Whipsnade expresses its complete form:
As my dear old grandfather Litvak said (just before they swung the trap,) he said "You can't cheat an honest man. Never give a sucker an even break or smarten up a chump."
However, Bartlett's Familiar Quotations attributes the saying to a remark made by Edward Francis Albee, who died in 1930. In any case, your cynical interpretation is accurate. The aphorism means "If a person is a fool, it is justifiable to take advantage of that person."