Coming from someone who has studied ethics and morality on a graduate level, I don't believe this is really an ethical question at all, the way you have presented it.

The basic question can be broken down to, "Is there anything wrong with a person of one nationality or descent wearing clothing that might indicate he is of another nationality or descent?" The basic answer is no.

My wife, for example, is more or less Western European (Irish, French, German) with a bit of Lebonese by way of Colombia. Yet she likes Japanese kimonos. She has no connections at all with Japanese culture. She just fancies that style of dress.

I don't imagine anyone would begrudge her that fashion choice. Why would it be any different for an American man of English descent who wears a kilt?

Now, if you were attempting to pass yourself off as being Scottish when you are not, that would be unethical. But in this case it would be the lying that was unethical, not the act of wearing the kilt itself.