it’s the same joke as “caesar salad was invented by julius caesar”

Except it’s a bit different because the Caesar in Caesar salad (named after chef Caesar Cardini) is actually spelled the same way as Julius Caesar, whereas Neapolitan (meaning of Naples) is not related to the name Napoleon at all.

It hadn’t occurred to me that Neapolitan ice cream might have something to do with the name Napoleon before I saw this meme; the similarity of the words and incorrect implication that they are related is what makes it funny.