Is it wrong to put on a fake German accent every time we order lunch?

It is not a major issue, but yes it is wrong. You are making fun of someone, and that ain't right. Even though he may not know it (uming he has always heard you talk like that, he may not know it is a fake accent), it is not proper. I think you know the answer. Give him a nice tip and cut the funny stuff. Now go and sin no more.