James from London
International Treasure
Pam thought it was better to keep Jimmy and Lucy apart so she specifically didn't want him wearing anything to impress Lucy. Pam cared about Lucy and was trying to help her, but though she'd make a terrible girlfriend for Jimmy. That's why he had to wear the boring shirt. Pam knew Lucy went through men like Kleenex and didn't want Jimmy to end up as just another one of many.
I'm pretty sure Pam was helping Jimmy land Lucy. She gave him the thumbs up for "acting like a hayseed and all" saying, "Lucy does not respond to 'cool'", cool presumably being the Roger Hurley type. She knew Lucy would be more intrigued if she had to work a little harder to get Jimmy, and so it proved.