I thought I'd provide some context for Maus and Art Spiegelman to help you formulate a thesis statement. Firstly, let's cover a fundamental aesthetic about the graphic novel, in which "image" and "text" are intended to be read as one unit, carrying the narrative as a whole. This effect is, to some extent, like subtitles on a foreign film: at some point, the dominant visuals and the captioning merge in your mind to create an impression. Spiegelman is of the generation of Underground cartoonists that started to break from the mainstream commercial assembly line, and he and his kind have striven to seamlessly integrate text and image into a holistic set of meanings.
That being said, in Maus, it's the visual symbolism that both distinguishes it and places it firmly within the tradition of anthropomorphic political cartooning. Actually, the portrayal of depicting humans as beasts—mostly derogatorily—hit a peak, in terms of concentration, during WWII. A famous example is the French children's book La Bete Est Morte! (1945), illustrated by Calvo. The cover features the iconic Hitler caricatured as the Big Bad Wolf. Another thing to think about as you formulate is, while it's obvious what the main animal symbols are in the cast, what are the exceptions? And what is the statement Spiegelman intends to make when he shows an exception, such as in the visual pun of wearing a mouse mask? How does a character switch from human to animal for the purposes of the meta-narrative?