Oscars

Apparently, some people thought that Ellen DeGeneres' joke pretending that the real Liza Minnelli in the audience was a female impersonator was offensive. Why?

There is nothing wrong in being a drag queen or other female impersonator. They are performers who want to be noticed.
And, certainly, it is not bad to be the subject of impersonation. The fun is that it is often Liza or some other star.  I thought that it was a funny bit in a really good performance by Ellen.