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.