You know what they say: if you can't beat 'em, make fun of them for their lack of masculinity by implying they don't conform to traditional gender norms!
I can't find any pictures of Luck actually wearing skinny jeans after a bit of quick Googling, so I'm thinking this is just casual bashing of a person who is good at football with a completely irrelevant and baseless charge. You are so cool, USC fans, with your antiquated use of the old "emasculate an opponent because football is only for manly male men" tactic. That is just a tremendous response to the Stanford band mocking your pornographer alum and consecutive victories by the Cardinal, bros. You rule.
Now, implying that Luck is a "tree hugger," as the sign behind the skinny jeans sign likely does? Considering Stanford's mascot and the Bay Area's typical political leanings, that's actually kind of funny.