Although the title may refer to the action within the film rather euphemistically as being "mean," Mean Girls is ultimately a movie about bullying. Upon starting her first day of public high school, Cady Stanton must come face-to-face with The Plastics and their cruel leader, Regina George, the most popular and nastiest girl in the school. After Regina sabotages Cady's attempt to get together with Aaron Samuels, Cady decides to get revenge with the help of her outcast pal, Janis, by targeting Regina's relationship, friend group, and self-image.

Although it may appear that Cady is seeking justice for the acts of cruelty that Regina has committed against the school, she is actually just participating in the same sneaky bullying tactics that Regina has used to exert control over the student population. The rest of the movie examines this power struggle and the ways that Regina and Cady's brutal behavior escalates. In this sense, the movie is also one about friendship, loyalty, and the cost of betrayal.

Other major themes of the movie could include the effects of peer pressure and fitting in at all costs as well as coming of age/rite of passage that the main character goes through.   Cady is a new girl at school who must find her way through the harrowing experiences she encounters with the mean girls.  It is through these obstacles and hardships, the decisions that she makes, and the lessons that she learns that she is able to grow and mature into more of an adult.  She also needs to figure out how much peer pressure is going to dictate her life.  Will she be an individual or desperately want to fit into the crowd?  Both of these are possible themes and are two subjects that are often found in teen angst movies like Mean Girls. 

This is a great question. There are many themes in the movie, Mean Girls. In light of this, let me name a few of them.

First, there is a theme of the horrors of being mean. The queen bee of the school, Regina George, played by Rachel McAdams is the meanest of the girls. She thinks that this is the path to happiness, but this causes more harm than anything. In light of this, one of the themes is that meanness does not pay. 

Second, Cady, played by Lindsay Lohan, thinks that she is above all of this, but she gets sucked in as well. When she becomes the most popular girls, she ironically turns into another version of Regina George. This shows that power corrupts people. Or we can say that there is a mean streak in all of us. 

Finally, there is some sort of reconciliation. This shows that this is the best way.