Not at all. You're supporting your team and your favorite player. If they made those for gymnastics, I'd be all up on it! I can admit I'm not half the man Paul Hamm is, and I am proud (for him) of his talent. And I am proud to have him support our great country, and that would be a way to show my support.
yeah it kind of is, I always want to go up and mess with people like that. Like, if a guy had a jersey that said payton manning, I'd be like "OH MY GOD, are you payton manning?! can I have your autograph?" then just laugh and walk away.
It does show one of your hobbies which may inspire conversation.. The con being that, in this world, there's truly less and less of a place for hobbies and passions. As baby-boomers approach retirement, less and less adults have access to IRA's and other traditional retirement funds, in turn it leaves a high expectation for them to work until their last day on earth. As such, our elders are holding a lot of job positions that new grads are continually fighting for. Unless you are already ahead, or have merely satisfied your own requirement for personal growth, it seems a little adolescent to express yourself in a way second to attempting to shed the best light on personal growth. Hobbies and passions are awesome in my eyes and I personally disagree with your friend, however, it does make sense in my mind that she might be coming from the perspective that she's worried about her future foundation and success.