“We’re gonna make you sing, Bart Simpson.” – Sherri
“Yeah, Bart Simpson, we’re gonna make you sing.” – Terri
When “Homer’s Odyssey” was first broadcast, and The Simpsons was considered just this side of Satanic cults by much of mainstream culture, one of the best things about it was the way it mocked success. People who did well in Springfield didn’t always deserve it, and even the ones who did were often portrayed as insufferable jackasses. The most glaring example of that is easily Flanders, who is a genuinely nice guy but who is also grotesquely inhuman in the way he is immune to the humdrum failures and humiliations of ordinary people.
Sherri and Terri, though much less prominent than Flanders, fill a similar role. They are goody two shoes; teacher’s pets who are plenty willing to abuse their favored status among the adults to torment Bart Simpson. They are proof that the kids who get good grades, do their homework on time, and never get detention can be just as mean and troublemaking as anyone else.
Just as bad, both they and their father, who’s one of Homer’s bosses at the nuclear plant, aren’t above using their favored status to shame and taunt people below them. In short, it isn’t enough for their family to be better, they have to rub it in. Society’s betters are just as bad as you are.
Like Milk Duds, they’re poison on the inside.
This idea of universal mockery is one of the things that differentiates The Simpsons from regular comedy, then and now. Just having Homer crash his cart and get fired in front of his son is funny. But even in Season 1, that wasn’t enough. Homer and Bart losing is much better when we not only see other people looking down on them for their failure, but also the way that the people looking down on them are selfish jerks too.
From the time they deliberately misinform Bart about US history to when they narc on Milhouse’s secret birthday party all the way up to trying to make Moe sing the million dollar birthday fries song twice, Sherri and Terri enjoy picking on people who aren’t as competent and put together as they are. They have a mean streak, and they’re perfectly willing to exploit the fact that they’re twins to express it.
The twins enjoy the suffering and humiliation of others. Just like the rest of us.
Unlike Zombie Simpsons, which frequently has characters show up in a scene for no reason other than to spout some piece of hacktacular dialogue, The Simpsons made even very minor characters like Sherri and Terri into real people. It understood that even characters who only get a few lines can be recognizable people, and that no one is too minor to have some funny flaws.