He's an extraordinary fuckwit by nature of the power into which he has stumbled. Paraphrasing the Bard, some are born fuckwits, some achieve fuckwittery, and some have fuckwittery thrust upon them. BoJo is a triple whammy of fuckwittery. Said as an observing American who has very much against her will become an expert on fuckwittery up close.
In his way, Jeremy Corbyn can be seen, in hindsight, as a sort of canary in the coalmine for Brexit and Johnson, and as part of the wave of populist discontent with the status quo.
After Blair, and then Brown and the 2008 crash, Labour really wasn't sure where it wanted to go. I was never really sure what Ed Miliband stood for, and when he lost in 2015, the contenders all came over as deeply uninspiring New Labour apparatchiks, except for Corbyn, who had been persuaded to stand simply so the left had some representation in the contest.
Party members weren't really sure what they wanted, other than "not more of the same," and MPs weren't really sure, either, other than "not Jeremy Corbyn," and things went downhill from there.
Corbyn doesn't really seem to have changed his mind on much since the late 1970s, and he'd always been happy as the left-wing rebel, being able to keep his political purity while others got on with the business of running the party, trying to win elections, and sometimes running the government, and it didn't go well.
Add to that he's not the brightest of people, he surrounded himself with some very dodgy advisors, and he was very happy to promote what many -- me among them -- saw as a shameful form of left-antisemitism in the guise of anti-imperialism, wrapped up in a perfectly legitimate concern for the Palestinians and dislike of Benjamin Netanyahu, and you can see why things went so badly wrong.