The feud between 50 Cent and Floyd Mayweather is still going -- Fiddy just CLOWNED the boxer's newly announced fight, saying his opponent is more "Uber driver" than contender.
As we previously reported ... Mayweather just signed a deal to fight on New Year's Eve in Japan against an undefeated 20-year-old kickboxer Tenshin Nasukawa.
Unclear whether the bout will be a boxing match or full-on MMA ... but, either way, 50 says it's a joke.
"Champ fighting a Uber driver before the IRS embarrass him again.