Granted, The Simpsons has perhaps overstayed its welcome. I have not watched it in years, and no matter how good new episodes might be, I would rather the show stay great in my memory. I don't want to see Conan with a walker, and I don't want to remember The Simpsons as a show that sucked.
That being said, Lisa is the archetype of the young nerd girl. She is obsessed with good grades. Her zeal for school work annoys her ambivalent teacher. She respects her principal (I never did that). She is interested in Zen Buddhism, literature, philosophy, jazz, the environment, and is a vegetarian and feminist. Her intelligence makes her question the world around her, particularly when she discovers that her beloved Malibu Stacy is a poor role model for young girls. She is the foil for her June Cleaver mother, her borderline retarded father, and narcissistic brother. Being a nerd girl, she is cursed to be loved only by the less-than-bright dork Milhouse... Her's is not an easy road.
You could write a book about Lisa, so I'll stop here and simply thank the Invisible Pink Unicorn (blessed be her holy hooves) that she has graced our televisions.