We don't see much of Max's mom. Actually, we don't see her at all. But we can learn quite a bit about her from the two words she speaks, how she responds to Max's anger, and her very last action in the book—delivering Max's dinner.
Even though she remains "offstage," so to speak, Max's mom does get a line of dialogue. When Max is out of control, she calls him "wild thing," which is what Maurice Sendak's mom used to call him. And when he sasses her, she sends him to his room. But it's what she doesn't do that is even more telling. Max's mom doesn't spank him, yell at him, lecture him, or tell him to wait until his father gets home.
All of this suggests that Max's mom is calm and competent, and his immediate compliance lets us know that she is firm and respected, too. And effective. Because the timeout she instructs Max to take? It works. Clearly, she knows her son and his moods well, and her last action—which again, we don't see, but we know she does—is to bring Max his dinner while it's still warm.
The calm smile on Max's face in the last picture says it all: Max's mom rocks. And she loves him best of all.