SPOILERS for 405 * * * * * * * * * * * * * * * * * This episode was primarily character development.
The big one was Jane's attempt to fit in, which she was actually enthusiastic about. Jane's always wanted to be accepted but been unable to fit in. She's slowly been expanding her circle of those who accept her for who she is: first Trent, then Daria, and now Tom. This episode gave her a chance to "try on" the role of being "popular".
Note how everyone who already liked Jane reacted to the "new Jane" ... namely with horror. Daria was utterly shocked to see Jane's "new look." Trent felt that Jane "really needed help." Tom was actually repulsed (this is understandable; he probably fell in love with Jane precisely because of who she was, and seeing her try to be somebody else would be very upsetting to him).
In the end, Jane realized that the price of conformity would be that her whole life, even in its most intimate details (friendship and love) would have to be spent in the company of people she despised. She chose to stick with a few people who really love her for herself over many people who would approve of her for a facade. Which was healthy.
The frailty of Kevin and Brittany's popularity was shown. Kevin is "nothing" without being "the QB"; Brittany, Mack, and perhaps Daria are the only people who really like him for himself rather than for his playing ability (Note that Mack stuck up for Kevin; he certainly didn't have to).
Likewise, all it took was a few depressing comments from Brittany for her fellow cheerleaders to desert her. Kevin, Jodie, and Daria are her only real friends. (Note that Jodie stuck up for her, and also didn't have to).
Daria actually bothered to help Mr. O'Neill. I always got the impression that she didn't hate him; now I'm pretty sure of that. I think she rather feels sorry for him, actually.
Daria's quest for failure was amusing but didn't really reveal anything about her that we didn't always know.
It's nice to see that Ms. Barch still likes Mr. O'Neill after the events of "Anti-Social Climbers". Her picture in his house was a cute touch :)
Sincerely Yours, Jordan