I don't like it and wouldn't wear it but I personally do not find it offensive. I could see a shirt like that being worn a lot in the States after Olympic events or July 4th celebrations and people having a good laugh over wearing them.
My guess is there are segments of American society that would find it offensive.
I also would never wear that outside of America. It not only helps to support the obnoxious, overbearing, uncouth, "cowboy" stereotype of Americans that pervade this world outside of America, but also rubs other people's noses in the fact we may have defeated their country or killed an ancestor of theirs in a war. Probably not a good idea.