Perhaps I misjudged the British perspective of Johnson; don't people see him as a pompous aristocratic clown? A cringing embarrassment, surely? To hear that he was the favorite to win before this is... alarming.
Yes you have misjudged. Johnson is pompous and clownish but he (unlike most politicians) virtually always has positive approval ratings. His final net approval rating with Londoners (who are significantly to the left of most of the rest of the county) was +23.
The problem with Boris is he is very good at pomp and show but very weak on actual substance. Thus he was pretty popular (and still is) when he was more in a symbolic role than one of power. However if he actually became prime minister some substance would actually be required which he would fall down on badly and thus his approval ratings then would probably collapse.
