Do you mean “I don’t get it?” In this example I think the person is saying that because he is American he finds something culturally unacceptable which someone is doing. ‘Take it down a noch’ Means ‘calm down’ or to be less extreme or ‘stop being so melodramatic’ it is an order to the someone to change their behaviour to be more calm. So in this context the American person is telling someone they are behaving in a way too extreme or dramatic for what he thinks Americans would like.