Not wrong at all, I'd say.
The only reason I can think of for thinking otherwise is that it would amount to not being loyal to one's country. We can agree that there are at least some kinds of loyalty we can normally expect from a good citizen. (Not committing treason is the most obvious example.) That said, it would be very bad if the demands of loyalty went all the way to which side you root for in a sporting match. That would be well down the road to mindless jingoism.
In one way it's a small point, but it has some real-life relevance. Noisy, thoughtless accusations of being "unpatriotic" are a far-too-familiar part of political discourse. If we worry that rooting for another country in a soccer match crosses the line, then the worry that we shouldn't disagree with any of our country's policies will seem all too real. That, however, is a disaster for thoughtful citizenship.
So root for the team of your choice. Root for them because they're the underdog, or because you like the way they play, or because you like the color of their jerseys. It is, after all, just a game. And you are, after all, not just a citizen of the United States but also of the world.