I don't think that is true at all. I think Mifune is dead on with his comments, in some respect.
The tone of the Indiana Jones films changes dramatically from Raiders of the Lost Ark to Last Crusade.
Raiders was a pretty badass film that was short on over the top humor. It had some dark humor, such as when Indy just shoots the guy instead of fighting him, but it lacked almost all of the slapstick type humor found in Last Crusade.
Last Crusade flat out made a mockery of some of the characters. Marcus Brody was a complete sham in Last Crusade. In Raiders, you had a guy who would go out and kick ass on his own if he were fit enough, then in Last Crusade, he is suddenly a bumbling fool who gets lost in his own museum and seemingly knows nothing about a profession that he has worked in for most of his life.
The comedic tone of the films definitely increased as the series progressed. I don't know if that is a good thing or not. Last Crusade definitely kicked ass, but to say that its type of humor is representative of the Indiana Jones franchise is a gross overstatement.