Eva, People will have different personal reactions to the term " 'merica" and those will also be appropriated to the context it was used in. The overwhelmingly negative reaction to the phrase that you witnessed likely wasn't due to your teammate's being called a 'merican, but rather that your other teammate implied a generalization of Americans, specifically that they are dumb/uncaring for laughing at their own mistakes. Likely the insulting party was frustrated to begin with and lashed out with a sh!tty joke, but the insult stuck.