im not a huge fall out boy fan, i dont listen that much but dont mind them. kids will get over the craze, then people will stop yelling that they sold out blah blah blah. people love to hate on the bands that become popular. a year ago, fall out boy would be the cool scene band to like, and all american rejects would be just another emo band. in 1998, blink was the cool scene band to like. once enema came out, people heard whats my age again on the radio and everyone cried sell out and then it became cool for music "purists" to hate. basically now, if you're not 15 and a teenage girl its not cool to like fall out boy. but it doesnt really matter, there will be another band like this to come along soon, and then in a few years, people who liked fall out boy from the beginning will band together and be the cool retro thing because nobody cares about bashing them because there is a new band that "sold out."
incidentally, i think the song about having good fashion sense theyre kind of making fun of the "scene" kids who like to dress like them and try to be like them. which is even funnier. props to a band that can make fun of their fanbase and then have even more fans.