It is an old joke, but perhaps the question behind it is worth exploring.
Claims have been made for both Tintin and Hercule Poirot. While I am firmly of the view that Poirot is the more famous of the two, at least in the English speaking world, Tintin did appear rather high (second, I think) in a poll of French people voting for the most important person of the Twentieth Century. It seems the French had a shortage of both French people, and real people, to vote for.
It is also worthwhile, when assessing the nationality of fictional people, to give some weight to the nationality of their creator, and Hergé was Belgian while Agatha Christie was not.
In terms of real Belgians, it really ought to be Georges Lemaître. He is not as famous as he ought to be, which seems odd. Why is he not as famous as, for example, Copernicus or Galileo?