I enjoyed reading this Reddit thread about celebrity geniuses exposed by misfortune, miscalculation and misbehavior to be "idiots". The top answer will be familiar to Britons of a certain age: Gerald Ratner, who built out a huge budget jewellery chain only to deliver a funny but catastrophically misjudged speech, whose chummy, insidery context was immediately collapsed by the press.
Sam Bankman-Fried, in number 2, was hailed as a philosopher king by glossy magazines and midwit pundits, until the inevitable collapse of the crypto grift him and his young pals were running on investors. This poser is on trial right now.
A sign of the times: the thread is centered on business executives riding hype and running into harsh realities: John McAfee comes in next ("Not sure of the genuis part, but the downfall was legendary"), then Elon Musk ("probably an idiot the whole time but he had the veneer of a genius for a while"), Elizabeth Holmes ("fake it til enough people ask tough questions"), and so on. All of these were cast as geniuses by media complicit in the whole cycle, from fawning to feeding frenzy.
Outside that genre, though, the top result is a good one: Linus Pauling, one of the greatest scientists of the 20th century, who ended up a vitamin quack and a key player in the Nobel Disease Wikipedia article (cf. Engineer Syndrome).