It actually is an ethnic slur.
There are thousands of insulting words that can be used to call someone "stupid".
The choice of a surname from a movie about Italians is, in fact, intended as an ethnic slur.
Or are you, with a straight face, honestly trying to convince us that "Fredo" is just some random name used to refer to a stupid person?
"Fredo" is, with certainty, an insult used to slur an ITALIAN person who is being accused of stupidity (made even more transparent by the fact that Cuomo is not stupid).