The character is just seen wearing it; there is no plot related to it. It makes no sense that he would go out of his way to have it made, as he has absolutely no interest in physics. Maybe he could have stumbled on it in a second-hand store and liked the design.
I just wondered if there was a real-world trend of Californian physics grad students wearing Peskin and Schroeder T-shirts, which might at least have slightly raised the probability that a Hollywood costume designer on a comedy show that has nothing to do with physics could have seen one in the wild, and liked it enough to decide to dress a character with no interest in physics in such a T-shirt.