Mrs. Shandy is Tristram's mother, and she's kind of out of this world. Really—she's part of the novel's secret world of women. Men and women have so little to do with each other that Mrs. Shandy has to spy at the keyhole to find out what her husband is talking about, and their conversations are pretty much one-sided. As Tristram says, her way was "never to refuse her assent and consent to any proposition my father laid before her" (9.11.7). She's sneaky that way. Take a look at her strategy in the conversation she has with her husband about getting Tristram some pants: "We should so," "It would so," "indeed," "exactly," "nothing can be better," "by no means," "I mean so too," "order it as you please," and, last of all, "perfectly … if it pleases you" (5.18). You're not fooling us, Mrs. Shandy.
Who wouldn't want a wife like that? Mr. Shandy, that's who. Her agreement outrages him, but she just goes on agreeing. But look at it this way—she's married to Mr. Shandy, the most disagreeable guy that ever lived. She probably figured out years ago that compliance was the easiest route to happiness.
At least she puts in some major gal pal quality time with Susannah. Mrs. Wadman's also got her back in a pinch. Susannah and Bridget, Mrs. Wadman and Mrs. Shandy: we don't ever see these pairs by themselves (Tristram Shandy majorly fails the Bechdel test), but, way more than Mr. Shandy, they change the course of Tristram's life.
Remember Mr. Shandy's decision to hire Tristram a tutor and put him into breeches? Mr. Shandy suspects that Tristram has spent so much time with women that he'll never fully learn how to enter an adult male world. (Tristram's difficulty telling a story suggests that Mr. Shandy might be right about that.) Mrs. Shandy represents a secret, almost frightening female world. Women are manipulative, sexy, and irrational.
But you know what they say: can't live with 'em, can't live without 'em. After all, no matter how much Mr. Shandy insists that women have very little to do with kids, they're the ones who make reproduction possible. And their reproductive potential turns out to be a powerful weapon. Mrs. Shandy's uterus is her only tool for getting what she wants, and it's the only way she can convince her husband to take her to London.
She's got the upper hand, all right. Mr. Shandy worries that if Mrs. Shandy goes to London to have her babies, it would "infallibly throw a balance of power, too great already, into the weaker vessles of the gentry … [which] would, in the end, prove fatal to the monarchical system of domestic government established in the first creation of things by God" (18.58). Can you say paranoid much?
What might be worse is that women throw off the whole narrative every time they show up. Mrs. Shandy literally prevents conversation by agreeing to everything. Mrs. Wadman distracts Toby with her sexy, sexy eyes. And then we've got Mrs. Shandy's famous question—you know, the one that sends Mr. Shandy's sperm awry.
She also asks the question that allows Yorick to end the book. In fact, you might even say that she begins and ends the book. So who really is more important to a child—mom or dad?