no one is stopping you. Just like no one is stopping an American posting American stuff, you can probably safely assume that not everything posted online is about you or to you directly. Feel free to go about your day posting what you want to. You don’t need an American’s or really anyone’s permission to do so. They didn’t ask your permission to post. Nor should they have to.
Real talk, I suspect there are a number of contributing factors to that phenomenon that have very little to do with John American sharing a twitter screenshot with Lemmy. I mean, Americans are just online, talking to each other. If we're that overwhelming, I wouldn't point any fingers at normal american people.
Americans dominate the English speaking Internet, because there are more English speakers. There are sites in German or Hindi that have almost never used by Americans.