This is a GREAT movie. I wish that I could show it to my students for the opportunity to reflect on these same qualities.
Idiocracy is, on the surface, just a crude comedy. When you look more closely at the "fiction as social commentary" aspect of the movie, it's frightening. The movie basically insinuates that if society continues at its current state, we'll doom ourselves to being "stupid".
In the movie, the educational system is continually disregarded and the standards continually reduced, so that in a mere 500 years, the entire structure disintegrates into a world where the IQ test for job placement is putting the square peg through the square hole instead of the circle one. As an educator, I can see this in day-to-day practice. Behind the scenes of education, there are a lot of choices made on politics and statistics that many of us feel do not benefit students on a long term scale. Perhaps you, as a student, have seen evidence of this as well.
The movie uses the devices of irony, sarcasm, metaphor, and satire to explore the social commentary being presented. There's a dramatic irony involved throughout because we (the audience) know what's really happened to "Not Sure" and we know his situation, yet we watch him struggle with trying to reason with this new society in vain, knowing that he's right.
One of the most obvious examples of metaphor comes with the use of the Costco as the answer to everything. The movie reflects today's society's attitude toward mass commodity and the "get it all in one place right now" demand that we make today (also satirical). Again, the warning that if we continue to act this way, then this is what we could be setting ourselves up for in the future.
There is satire in the corporate brainwashing that goes on in the movie, too. Brawndo, for example, is supposed be what plants crave, and the corporation has used propaganda to convince the society that because it's got electrolytes, it's therefore better for plants than pure water, which has been degraded to use only in toilets. Think about the push we have in our society that focuses on all of these miracle cures and fast fixes. Think about the ways that our media and our corporations market their products to make people feel worthless if they don't use them.
I could go on and on about this movie and the powerful message I think it carries, but there's a character limit in this little box, and I'm not supposed to answer the question for you completely. Hopefully, my meanderings here have given you some insight and can help you get your thoughts together to create your own analysis of these devices and the movie's message in general.
The themes of this movie are similar to that of the Huxley novel, Brave New World, and other dystopian ideas. Perhaps a look into that or More's Utopia could give you more insight.