(I was going to call this entry Who was the worst mathematician of all time? but Clyde Kruskal reminded me that its not (say) Goldbach's fault that his conjecture got so well known, in fact its a good thing! I'll come back to Goldbach later.)
Would Hawking be as well known if he didn't have ALS? I suspect that within Physics yes, but I doubt he would have had guest shots on ST:TNG, The Simpsons, Futurama, and The Big Bang Theory (I just checked the IDMB database- they don't mention Futurama but they do say he's a Capricorn. I find that appalling that they mention a Scientists Horoscope.) I also doubt there would be a movie about him.
Would Turing be as well known if he wasn't gay and didn't die young (likely because of the ``treatment'') would he be as well known? I suspect that within Computer Science yes, but I doubt there would be a play, a movie, and there are rumors of a musical. Contrast him with John von Neumann who one could argue contributed as much as Turing, but, alas, no guest shots on I Love Lucy, no movie, no Rap songs about him. (The only scientist that there may be a rap song about is Heisenberg, and that doesn't count since it would really be about Walter White.)
Hawking and Turing are/were world class in their fields. Is there someone who is very well known but didn't do that much?
SO we are looking for a large gap between how well known the person is and how much math they actually did. This might be unfair to well-known people (it might be unfair to ME since complexityblog makes me better known than I would be otherwise). However, I have AN answer that is defensible. Since the question is not that well defined there prob cannot be a definitive answer.
First lets consider Goldbach (who is NOT my answer). He was a professor of math and did some stuff on the Theory of curves, diff eqs, and infinite series. Certainly respectable. But if not for his
conjecture (every even number is the sum of two primes- still open) I doubt we would have heard of him.
My answer: Pythagoras! He is well known as a mathematician but there is no evidence that he had any connection to the theorem that bears his name.
Historians (or so-called historians) would say that it was well known that he proved the theorem, or gave the first rigorous proof, or something, but there is no evidence. Can people make things up out of whole cloth? Indeed they can.
Witness this Mr. Clean Commercial which says: they say that after seeing a magician make his assistant disappear Mr Clean came up with a product that makes dirt disappear- the magic eraser. REALLY? Who are ``they''? Is this entire story fabricated? Should we call the FCC :-) ANYWAY, yes, people can and do make up things out of whole cloth and then claim they are well known. Even historians.
Commenters: I politely request that if you suggest other candidates for large gap then they be people who died before 1950 (arbitrary but firm deadline). This is not just out of politeness to the living and recently deceased, its also because these questions needs time. Kind of like people who want to rank George W Bush or Barack Obama as the worst prez of all time--- we need lots more time to evaluate these things.