I'm not defending Joe Lewis, but he did take acting classes, and if you look back to that era (1970 -80s) there were only a few American MA who were able to get into movies.
BL, Joe Lewis, Leo Fong, Ron Van Clief, Jim Kelly, Benny Urquidez, Cynthia Rothrock, and a handful of others. I believe only Joe took any lessons. Most of Ron Van Clief's movies were actually filmed elsewhere usually in Asia.
Of those, some of them jumped on the 'Blacksploitation' bandwagon, and others on the Bruce Lee bandwagon,
It's true that most of the martial arts guys were not good at acting and many of the people in those movies.
It was a limited window of opportunity and it's hard to blame some martial artists for trying to cash in on their relatively brief fame. It takes a while to become skilled at acting just like any other and to wait would be to miss the opportunity.
I think I'd chalk up the failure of those movies he did more to bad editing, bad writing, and not very good action filming techniques. But I agree, that he was definitely unable to bring his charisma onto the big screen, and his 'golden boy' looks and martial arts ability was not enough to make those memorable.