I don't think you could answer the question of
worst World Champion. If we reverse the scenarios in which we praise under-rated drivers, or for those who deserved to be world champs, it's usually because they "were in a car that was not the best at the time", or something along those lines.
Formula One World Champions, love them or hate them (or in between), managed to put their skill and talents in the right place and at the right time. Some were shrewder than others to make sure their team was right behind them, or that they moved to the right team when the opportunity arose, but nobody lucks into a title without being faster and more careful than everyone else over the course of a season's worth of races.
Even in a case like the '82 World Championship, Keke Rosberg (who probably gave the best quotes and respect to the press in the entire business, before or since...and one of the driver's I'd like to meet in real-life one day) didn't overdrive the car, kept a level head throughout the season, and won his Driver's Championship fair and square.
You could draw up reasons for any "undeserved" title, for just about any season since 1950. Heck, people still argue who was the real
1939 European Champion of Grand Prix racing...