I liked her even when she was used to represent a progressive or spiritual viewpoint, because you can't do that with the other characters on the show. It makes a good dynamic and allows for some different types of stories, and it was endearing to see such an intelligent child grow and start to face these philosophical problems people usually run into when they're a little older.
But the more they used her for those kinds of plots the more serious they had to make her, I guess because they feel like people wouldn't take her moral conflicts seriously if they also had her being portrayed as a kid with kid interests. Also they made her smarter and smarter until she was some kind of ultra genius, and the more they went down that direction the more conflict with the family they were forced to include.
Unlike most here I don't think her turning into this obnoxious and unlikable character was necessarily about pushing a liberal or rationalistic message, at least not until the much later seasons, but rather because of flanderization like all the other characters. The way they kept developing her as an intellectual and moral foil to the rest of the family became more and more extreme as the rest of the family themselves got dumber and dumber.
