My personal opinion is that expressions like “world’s greatest” make it seem like there is an absolute scale by which all bassists can be measured, regardless the genre(s) they explored, instrument(s) they played, era(s) they lived in, and contribution(s) they made to the art.
I don't think there is an absolute way to rank the greats, so I don't think he was the world's greatest or wasn't the world's greatest, because the idea that there could be a "greatest" in some absolute sense doesn't work in my brain.
Now as for the portrait: I love it, and given that he referred to himself as the "World’s Greatest," I like the line on it. It captures who he was and what he thought of his contribution to music.
And isn’t that what portraits are supposed to do? Capture the soul of the subject? If reproductions are available for sale, count me in. I’d hang that in my house in a shot. With the claim, whether I think it’s meaningful to compare great bassists or not.