Why Jackie Chan is on Dragon Ball? In episode 21 of Dragon Ball, an old warrior called Jackie Chan (a fake name for master Roushi) was introduced.

Dragon Ball is from 1985 and this is one of the earliest episodes, was Jackie Chan even famous back then?

Is it a joke that only a few have understood or was it understandable to everyone?
Wikipedia notes that Jackie Chan's first breakthrough was the 1978 film Snake in the Eagle's Shadow.

He was already well known in the 1980s. His first Hollywood film was The Big Brawl in 1980.

Furthermore, we should note that dragonball is a Japanese series. From wikipedia,


  Back in Hong Kong, Chan's films began to reach a larger audience in East Asia, with early successes in the lucrative Japanese market including The Young Master (1980) and Dragon Lord (1982).


Emphasis mine.

It's pretty clear that he was at least somewhat well known in Japan at the time.