For the record, the book is non-fiction outside of drawing animal heads on characters. It's a true story of a guy telling the experiences of his parents living through it, and of him writing it.
The animal features are actually a great way to talk about the racial essentialism of the Nazis while pointing out it's meaninglessness.
I'm not sure that I buy the "it's a great way to talk about racial essentialism" idea, because having the germans be cats and jew be mice sounds like it frames the idea that jews are actually a fundamentally different race than the germans as correct and accurate.