Much has been written about Art Spiegelman’s Maus, both as a graphic novel, comic book and Holocaust angle. So, I won’t repeat what others have said far better than me.
I’m not sure why Spiegelman chose to render his characters as animals. Unlike many types of allegorical stories featuring animals, they usually stand for specific types of character types. And indeed, the Germans are cats, the Poles pigs, and the Jews mice, but this device is not overly used. We are not beat over the head with it.
It seems that the device was used by Spiegelman as a shield; a way to get at the very difficult topics he is handling. Proof of this is the comic-within-a-comic, which he wrote earlier in his career, featuring human characters. The Jewish mice do not comment of the change of species at all. It is simply not important.
We are to understand this world as ours entirely.