That's literally what happened. A book was published in which white house officials shared stories which questioned his mental fitness. The stable genius thing was from a series of tweets he sent attacking the book.
“Actually, throughout my life, my two greatest assets have been mental stability and being, like, really smart,” the President continued. “Crooked Hillary Clinton also played these cards very hard and, as everyone knows, went down in flames. I went from VERY successful businessman, to top T.V. Star … to President of the United States (on my first try). I think that would qualify as not smart, but genius … and a very stable genius at that!”