That's actually the way most people in Europe see him. Our source is not any opposition but Donald himself: listening to his interviews and speeches lets him certainly come across as "erratic" and "incoherent", he bragged about "sexually assaulting" women, and how he talks about and treats minorities makes him seem "racist" and "bigoted". As he has no clue about the state of world politics and no coherent policy proposals (flip-flopping during the same speech, interview or even sentence) the "idiot" label seems hardly avoidable. You could argue the "tax-dodging" though, for all I care.
I also don't see him as a new Hitler, more like a Mussolini type.
>just like the Republicans who claim Clinton is an "obviously erratic deathly ill man-hating murdering warmongering power-hungry alcoholic liar."
In Germany she would clearly be a conservative, which I'm not, but listening to her she comes across as extremely smart and very coherent, so "erratic" really doesn't apply. No idea why I would think she is "deathly ill", never saw anything that made her come across as "man-hating", no idea where "murdering" comes from; "alcoholic"? Never heard about drinking problems. That leaves "warmongering", "power-hungry" and "liar": maybe not more than the average American President in the last 200 years.