Even though Art Spiegelman drew all the people as animals it didn’t take away from the seriousness of the story in fact it added to it. The reason behind his drawing it this way is pure symbolism. Jews were symbolized as mice, seen by the Germans as vermin and lowly creatures. Police were represented as pig, which is pretty self-explanatory. Germans were quite appropriately drawn as cats, seeing themselves as superior, stronger, etc. It’s classic cat vs. mouse. Cats run the mice out and kill the mice; ridding the world of vermin.
The way Spiegelman tells the story makes it feel all the more real. He doesn’t simply tell and illustrate his father’s story but he tells it through his father’s voice and the way his father told it to him, tangents included. Stories are always more enjoyable when a character’s dialog is written exactly the way they speak and in Maus Vladek, Spiegelmen’s father, obviously has broken English. Spiegelman writes his father’s words, not his own, because it’s his father’s story, a true story, and when it’s purely Vladek the story is more powerful. You don’t get to allow yourself to forget that it’s true because of the manner in which it was presented.