Ignore humourless critics who pan this movie by comparing it to something like The Producers.... I don't even know why people use that movie as a comparison point.... The Producers was a cynical satire about the entertainment industry and uses a cartoonish portrayal of Nazis as a vehicle for to describe how ludicrous that industry and critical opinion of it can be.
The Producers was not a satire which analyses the thought processes of young children subjected to systemic state-enabled brainwashing.
That gripe out of the way, why Jo Jo Rabbit is a worthwhile film.
Costume and settings wise, there are many things which are technically accurate in the portrayal of the dying days of the Third Reich, a time when the German armed forces, the Wehrmacht, was down to its last legs and when child soldiers and rapid conscription of ill-suited civilians into it became a thing.
The way that Waititi portrays an imaginary Hitler is very old fashioned cartoonish, the children act like believable children who are only beginning to realise the historical freight train that will hit them, the humour does a good job of referencing things which were happening in Germany end-WW2, houses of characters felt lived in and even the uniforms of some of the characters manage to tell a story (for example, when Sam Rockwell's Captain Klenzendorf character first appears as the disillusioned Wehrmacht officer, his uniform indicates that he would have been a distinguished infantryman who saw extensive combat and who had taken out a tank).
In other words, care was taken to provide attention to detail and although the movie is not a documentary, it does drift between outlandish (sometimes even Monty Python-esque) humour and serious matters rather quickly, but it is the attention to detail that allows it to happen.
More importantly, and this is something I found very cleverly executed, this movie is about the evolution of a 10 year old within a dictaorship as opposed to someone with an expansive knowledge of what happens in WW2 let alone what it was about and how it turns out. This is where the movie shines.