"One joke" here means a funny event, or an absurd, contradictory situation. Yippies were counterculture activists from the 60's, and yuppies are "young urban professionals", that is, just the opposite from yippies

Well, I guess I can tell you what 'yippie' and 'yuppie' mean yippie means 'young international party' (YIP). They were a group of young people in the 1960s. They were like hippies, but different. Hippies were young people in the 1960s who tried to rebel against tradition by wearing different clothes, behaving differently, and sometimes using drugs. The Yippies were known for being more politically active. People just said, 'yippie' instead of YIP since it rhymed with 'hippie'. 'yuppie' means 'young urban professional'. I guess whoever, wrote the passage was perhaps saying that, later in life, the people who were 'hippies' and 'yippies' changed and became professionals. Ok. That's kind of a long explanation. I hope that helps.