Acceptable Targets: The cartoon insults Nazis generally, Hitler specifically. Could a target be any more acceptable? The racist caricature of Hirohito is arguably an exception, although certainly not at the time the cartoon was made.
Here's a bit of historic context — the idea of passing gas in Hitler's face would be considered somewhat crude nowadays, but back in the 1940s it was borderline obscene. If the face being farted in was anybody but Hitler's, the song would never have seen the light of day.
Special mention goes to the racist Japanese stereotyping.
And the third chorus mentioning people "working into their graves". Two years later, the world finds that hundreds of thousands, if not a million Jews, prisoners of war, and other 'undesirables' were used as slave laborers, many suffering that very fate, to say nothing of the millions simply killed outright.
One of the lines in the song is "Ist ve not the supermen." Superman later on fights the Nazis.
Especially funny if you remember that Millenium in Hellsing were Nazi vampires (i.e. "supermen").
While Donald is assembling the small bullet-like shells, we hear this line in the song: "If one little shell should blow him (Hitler) right to *CLANG!*" Two years later: he commits suicide by shooting himself.
Overshadowed by Controversy: Thanks to the internet, a lot of people have heard that there's a Disney cartoon where Donald Duck is a Nazi. Very few of them, apparently, know that it's a scathing satire of Nazi Germany and instead assume that it's pro-Nazi propaganda. Hence, the cartoon is often accompanied with a "Not Making This Up" Disclaimer.
Some Anvils Need to Be Dropped: The Nazis are bad, the USA is good. Thank your lucky stars and stripes you're an American. Don't take your freedoms and prosperity for granted.