If you want to mock someone's nationality, it's normal to make lots of references to stereotypes about it. It often doesn't matter as much what those are as that you make it clear that you're making fun.
However, if you want to mock someone's nationality to their face, you may have to tell them what those stereotypes are first. If not, they may not notice at all. If you have a large crowd of your own nationality to play to, you may be able to make it clear you're mocking them by all laughing uproariously whenever they say something, and actually make them feel bad they don't know what you're talking about. But if you're on your own, it may fall completely flat.
You may have a conversation like:
American: Have you eaten a lot of sweets lately? Haha!
Brit: Whu? Anyway, we were saying...
Which didn't really hurt anyone's feelings at all. You'd probably do better to step back a bit and make explicit whatever stereotype you're talking about. If the victim is unaware, they may actually be a lot more discomfited at facing a new stereotype.
American: Do you know your teeth are really ugly? Is that because you're too stupid to know how to take care of them, or too poor to have surgery?
Brit: *punch*
In fact, I'm pretty bored of stereotypes about nationality. But if you're going to do it, for god's sake do it right, ok?
However, if you want to mock someone's nationality to their face, you may have to tell them what those stereotypes are first. If not, they may not notice at all. If you have a large crowd of your own nationality to play to, you may be able to make it clear you're mocking them by all laughing uproariously whenever they say something, and actually make them feel bad they don't know what you're talking about. But if you're on your own, it may fall completely flat.
You may have a conversation like:
American: Have you eaten a lot of sweets lately? Haha!
Brit: Whu? Anyway, we were saying...
Which didn't really hurt anyone's feelings at all. You'd probably do better to step back a bit and make explicit whatever stereotype you're talking about. If the victim is unaware, they may actually be a lot more discomfited at facing a new stereotype.
American: Do you know your teeth are really ugly? Is that because you're too stupid to know how to take care of them, or too poor to have surgery?
Brit: *punch*
In fact, I'm pretty bored of stereotypes about nationality. But if you're going to do it, for god's sake do it right, ok?