lol.. I didn't feel like analysing the reasons for the comments.. I just think Mark and co. like to take jabs at Haley.. lol.
Lucas - I think it was more about his "unique and great" friend Haley conforming to the high school hierarchy. It wasn't "her", and it shocked him. Ya, he probably blamed Nathan to a point since he did say something like "she was the great unique Haley and then you came around"... He feared that Nathan was changing her, and her dressed as a cheerleader solidified that for him. He was an ass, but in his own mind, he was looking out for her.
Nathan (3.04) - well, he saw through the act lol. He saw what she was trying to do, and while I am sure he liked it, he knew that she was trying to be something she was not to get his attention. He basically said "she always looks hot" to make a point that she doesn't really need to try with him. He's gaga for her no matter what and more so when she's not putting on a show.
I mean, it could be that or it could be Mark has a vengeance on Haley.. lol. Who knows. . . .