Conversation so far:
Who did John F. Kennedy Jr.'s mom get married to?
Jacqueline Kennedy married Greek shipping tycoon Aristotle Onassis.
When did John F. Kennedy Jr.'s mom get remarried?
In 1968, Jacqueline Kennedy married Aristotle Onassis.
How did John F. Kennedy Jr. react to his mother's new marriage?
John F. Kennedy Jr. is said to have considered his stepfather a joke.

Question: Why did he think he was a joke?
Self-contained rewrite: Why did John F. Kennedy Jr. think Aristotle Onassis was a joke?