The grand fib of lessons learned has long been debunked. Even Jack Granastein, who tends to be pretty conservative in his historical interpretations, refers to Dieppe as a "colossal blunder".

"Back in England, nothing mattered except that reputations be saved. The Dieppe raid became a model of how to spin the facts, as historian Timothy Balzer has conclusively demonstrated. Mountbatten’s expert public relations team turned out to be better at preparing their media lines for failure than his planners had been at drawing up the strategy for operational success. Combined Operations Headquarters instantly claimed that the raid had been a great achievement and learning experience, and they held to that line even after the casualty lists became public." http://www.canadashistory.ca/Explore/Military-War/Dieppe-A-Colossal-Blunder