Mom doesn't get a whole lot of action in the story, but she seems to be around a lot of the time. She's the driver, the listener, the disciplinarian, and finally, in the end, the tell-it-like-it-is-er.
In fact, while Alexander represents all things boy, Mom breaks a whole lot of the stereotypes we might bring into the story. First, she's not super involved in all of the things that Alexander is doing. She doesn't act in any way other than to show up when Alexander is getting revenge on his brother, and even in the end, when she is the voice of reason, it comes in the form of Alexander's paraphrase.
Some people have come to think of this as an "ambivalent" approach to parenting—it's certainly not the all cuddles, hugs, oohs, and ahs that we might expect from, say, Joan Cleaver.
Not us here at Shmoop.
Keep two things in mind before passing judgment on Mom:
(1) All of this is told from Alexander's eyes, which are in turn tinted by the lens of an already terrible day, so...
(2) Mom's style seems to be one of encouraging more independence from her boys. She lets Alexander makes the mistakes he does, she takes on some authority when things get punchy (even though Nick totally started it), and her straight-forward message at the end—that some days are like this—is an oldie but a goody.