She did become a punching bag you are right. When people are ambitious and nerdy and popular they tend to be mocked by those who aren't haha.
But, I do think she really only turned into an ungrateful whiner at the end of season 4 when she left. When she blames Dan for the bike shop closing is cringeworthy. The first bit was tolerable, but the second half when she starts with "come on mother, you know it but I'm the only one with the guts to say it. You blew it Dad, you blew it big time."
That was way over the line in my opinion and totally unecessary. The point had been made.
Part of the blame for her behaviour has to go to the fact that they/Lecy were purposely trying to write her character out of the show so they amped it up a bit.