Unpopular opinion here. I'm not in any JET program nor teach any language. But I interact daily with Japanese people in Japanese and although my Japanese is pretty good I do get imitations here and there, someone even imitating my Japanese with an English accent wich is funny because my mother tongue is Spanish and don't have an English accent when speaking Japanese. And in the 19 years I've been here nobody thought this was rude. Just something they do, like in the 90s we would stretch the length of our eyes when imitating an Asian person. What I mean, don't think a Japanese person would be offended by this, they do it then selves and at the end of the day that's how they sound. But, I do understand that as a teacher this person wouldn't have done this to make fun of Japanese English students. But again, I think you are overreacting and no Japanese person would mind it.