The family matriarch, Úrsula, is a woman of indomitable will and bottomless energy. She heads the family, takes care of the giant house, runs a business, and lives to be over 150 years old. Not bad.
If there's anyone who knows about activity, it's this lady. At least three times in the novel, Úrsula picks herself up, dusts herself off, and sets out to completely renovate the giant house in which the Buendías live. In between these bursts of activity, she somehow manages to take care of a bazillion kids (pretty much every child that passes through that place!), cook for the family, and still somehow have time to run her candy business.
Úrsula might just be the closest thing we have to a purely positive character. She's a whirlwind who keeps the gang all together despite the Buendía tendency to drift apart into loner-dom. She is pretty rational and sane and tries to keep everyone's insanity in check. And she doesn't get hung up on crazy obsessions; she's way more practical, in the moment, and self-aware.
Because we have her example before us, it's all the more shocking when we see how much lazier and less active each successive generation of Buendías becomes. Because of this, we can kind of refine our sense of just how the repetition of history works in the book. The novel is circular, but each generation isn't simply a copy of what came before. A better image would be a downward spiral, kind of like that cool circular ramp in the Guggenheim Museum. Like the ramp, the family just goes down, down, down (into laziness, stupidity, and apathy).
Because the narrator of One Hundred Years of Solitude doesn't pass any judgment on the characters, perhaps we can see Úrsula as a stand-in for this kind of ethical commentary. She doesn't explicitly comment on the craziness of the rest of her family, but her presence alone serves as a reminder that normalcy and sanity do exist.