I agree with my colleagues that Diana Moon Glampers is a flat character.
By definition, flat characters are less developed, often two-dimensional, and sketchy at best.
Nonetheless, these characters are still important to many works of literature. In Harrison Bergeron, Diana Moon Glampers exemplifies the evils of government control. While she looks average (much like Harrison's mother, Hazel), she is far from equal to the other citizens in the story. She, in fact, has power over everyone. Her character is in the story to show the inequality and corruption of a communistic society. So, while she is flat, she is definitely important to the story.
I located a site that has a pretty good chart (along with some other information) about round, flat, static, and dynamic characters that may be of some help to you.
Round Characters are more fully developed
are undeveloped and two-dimensional
Dynamic Characters undergo change over the course of the story
Static Characters do not change.
Therefore, a flat character will likely NEVER be a dynamic character; since they are only the sketch of a character, they are never developed enough to undergo a major change of heart or experience growth like a round character can.
Interestingly, in many fairy tales and in some very short stories, there are NO round characters. ALL of the characters may be stereotypes with no real development other than their symbolic significance.