The in-universe explanation is simple. The Nazis did not exist in the past of Star Wars (since it takes place "a long time ago"). Therefore, imagery that we today associate with the Nazis would not have been anathema to the good-aligned rebels. They simply formulated their medal ceremony in a way that was intended to be impressive.
The out-of-universe reason is also simple. Lucas ha explicitly stated that Triumph of the Will was an important influence on his filmmaking style. (See, for example, this article in The New Yorker.) Leni Riefenstahl is still considered one of the most effective film directors of all time. As a person, her work filming in the Nuremberg rallies was damning, and negative feelings toward her persisted long after the Second World War; she did not get the quantity or kind of work that one might have expected for a director of her caliber. However, many filmmakers, including Lucas, cannot help but appreciate her skill and the visual effectiveness of Triumph of the Will. Essentially, Lucas just copied the Nazi propaganda look because it was a powerful look.
It is true, of course, that the original trilogy of Star Wars films also used Nazi imagery in connection with the Galactic Empire. The color of the gray uniforms worn by most of the imperial officers was not chosen at random; and the snowtrooper helmets are modeled after the German stahlhelm. However, Nazis were not the only source of suggestive imagery. For example, the bridge layout in the imperial star destroyers, with the most senior commanders walking above, while most of the crew toil below the level of their feet, was chosen to be reminiscent of slave galleys.