Catherine Tate seems to be an 'all or nothing' performer to me. Sometimes her characters are very funny, sometimes they fall flat as a pancake. She does have a tendency to stretch a one-joke character so thin that eventually it snaps and the original joke becomes so overworked that it just isn't funny any more.
"Queen of Oz" doesn't seem to have a joke or a character that hasn't already been bent out of shape to fit into a shaky framework that just isn't funny. Some will love it I guess, but it's not for me.
The whole premise is so far-fetched as to be ridiculous, it's neither based in reality or interestingly surreal, it's just one of those ideas that seemed funny when you were drunk one night, but really can't stand the harsh light of day.