In 1982, had Gilles Villeneuve or Didier Pironi actually been around for the entire season, one of the two of them would have won the title, not Keke Rosberg. The Ferrari 126C2 was clearly the class of the field(not to disparage the class of the drivers either).
Outside of this, however, considering that Keke Rosberg was up against the turbo might of Ferrari and Renault (having only a normally aspirated Ford DFV himself) he was able to score one win, five other podiums and four more points scoring finishes. Other than his three retirements and one disqualification, he only missed the points once. The next highest non-turbo driver in the championship, John Watson, scored two wins and three other podiums (so the same total of Podiums as Rosberg) but missed the points three times and retired four times. Again, considering the fact that the Renault either blew up or won, if Ferrari didn’t win, and Ferrari didn’t have a consistent driver lineup, I’d say Rosberg is a deserving champion based on what he was able to do with a underpowered car.
Beyond his championship year he won Monaco in 1983, again with a Ford DFV, when the only three championship contenders in 1983 were Brabham, Renault and Ferrari, all with Turbos. Between the three teams they won 12 of the 15 rounds. In 1984 Rosberg won one race, his team-mate won none, and Rosberg’s Williams was sixth best on the grid by constructors points. In 1985, Rosberg tied his team mate Nigel Mansell with two wins a piece, and outscored Mansell by nine points. 1986 is the only year Rosberg can be brought to account for, having no wins to his team-mate Prost’s four, and scoring 50 points less than Prost. However, Rosberg had eight retirements, hence, his suffered poor reliability. Additionly, though not to make excuses, the only team-mate that has ever beaten Prost on sheer pace was Ayrton Senna , and that’s saying something for the abilities of Alain Prost.