So he struggled with a Middleweight puncher. Lost to a Middleweight Boxer. He won the Heavyweight champ against an Umpaloompa. Refused to rematch the REAL Heavyweight King. He was just a place-holder for Luck McCarty. Had McCarty put his Cowboy lifestyle aside until after his pro-career Johnson, and possibly even Dempsey, would have had only a marginal place in Boxing history.
I think it is common knowledge that Johnson was a dissipated fighter,31lbs over the weight he had bested Burns for the title,and undeniably past his best, whereas Willard was a fresh, prime , motivated challenger without the mileage or lifestyle on his clock that Johnson carried . Whether Johnson qualifies as one of the GOAT probably depends on whether you take into account his ground breaking emergence as a black heavyweight champion,prepared ,like his immediate successor, to put his title on the line in foreign climes,and also if you think his style translates to modern boxing as we know it.Personally I put him above his predecessors and in the mix with Dempsey Louis,etc, others can arguably exclude him on his lack of work rate,and clinching style.