When Family Guy debuted 20 years ago, it was built up as a subversion of more traditional sitcoms. The animated format and slapstick tone allowed for the show to go as big as it wanted to, and it played with the characters accordingly -- at first. They quickly became hardened parodies of the more cliched characters in sitcoms. For example, Peter Griffin's "dumb but well-meaning" father archetype morphed into an idiot who ruins everything while being juvenile and unrepentant. The show as a whole doubled down on this, turning every character in the series into the most extreme version of themselves, all for the sake of mining those traits for the frequent dark jokes the series employs for shock laughter.
But while Family Guy has turned almost every character into a one-note joke machine designed to feed into the over-the-top offensive bits that fuel the series, Stewie Griffin has actually become increasingly nuanced and compelling. Seriously, he might be one of, if not the most complex character in modern animation.
Stewie was introduced as the most cartoonish member of the cast, even more so than Brian, the talking dog. Twisting the traditional perception of children as characters, Stewie was in no way cute. He was homicidal, casually cruel, and generally just a super villain in diapers. The joke was watching him have to interact with the more mundane world around him, like preschool or the playground, and how his evil tendencies changed those aspects of his life (his primary goal was to kill his mother, Lois). But as the series continued and the creators grew more bold with their stories, Stewie started to develop a soft side.
His genius intellect and dark morality didn't stop him from actually enjoying things like his favorite children's show or trying to make friends. He quietly began to appreciate things in life, even indulging in just acting like a child at times. It's in those moments where Stewie actually seems happy, but they're few and far between. He cites the fact that he's smarter then everyone he knows, not just technologically but socially and maturely, as a reason to act like he hates everything. He already knows life will disappoint him, so why bother? His overt and forced cynicism has even been commented on in the show, with his brother Chris at one point verbally stunning Stewie by bluntly asking why he has to always be so negative.