Team America is funny, not offensive (although in the end it tends to justify this nonsense notion of the US as the World Police).
Is this shirt trying to make fun of people who agree with its message? I'd find that funny, but I guess a lot of people in the US would take offense on that.
Seriously, I still don't get the use of "America" as a synonym to United States of America. It's not like the name didn't exist before 1776, right? My guess is people from the rest of the continent might find some disrespect in that.
Finally, CelticConcourse is right, there's nothing to brag about wars. Of course somebody wins, but common people always pay the price. It would take a lot of space to discuss this subject, so I'll leave it there.
Every country has its sterotypes, and it's good to make fun of them, we need more humor and less wars.