Unfortunately you haven't provided details of the article in question nor have you quoted any particular passage. I presume that the article you’re referring to is the one dated 6th July 2023, which begins:
Robert F. Kennedy Jr. is a crank. His views are a mishmash of right-wing fantasies mixed with remnants of the progressive he once was: Bitcoin boosterism, anti-vaccine conspiracy theories, assertions that Prozac causes mass shootings, opposition to U.S. support for Ukraine, but also favorable mention for single-payer health care. But for his last name, nobody would be paying him any attention — and despite that last name, he has zero chance of winning the Democratic presidential nomination.
If this is indeed the passage you had in mind, I don’t agree with you that Paul Krugman is using the word ‘crank’ “pour suggérer l’idée de crétin” (although he clearly doesn’t have a very high opinion of Mr Kennedy).
The word ‘crank’ is used by Paul Krugman in exactly the way the word is defined by the WordReference English dictionary: "a person who has strange ideas or who supports bizarre causes”. The Cambridge Online Dictionary defines a crank as "a person who has strange or unusual ideas and beliefs.” The French translations offered by the WordReference dictionary - excentrique, hurluberlu - are, in my view, rather misleading in this context. They suggest an oddity or bizarreness of behaviour, appearance, speech or mode of dress, none of which is directly relevant to the word ‘crank’, as used here.