Maybe the electorate wouldn't have been quite so "gullible" if the only alternative choice had actually run a competent campaign?
Interesting, too, that you should call out Corbyn for being not what his followers thought what they were getting, when much the Tory manifesto is likely to be discarded in like manner. At least Corbyn was never going to be able to fail to deliver on his promises.
No, he was underestimated. Two years, almost, he led the Labour Party, and not a day went by when he wasn't vilified or criticised -- often justly, just as often way overdone. And he was described as unelectable, and despite the loss leading Labour to their largest vote share since 1997 puts paid to that lie also.
As for your continuing contempt of the electorate... you ought to try appealing to them, rather than dismissing them. Hard to win elections if you ignore or annoy the people who have to vote for you, isn't it?