Some dweeb on a Rotten Tomatos forum did not understand the greatness that is "The Chronic of Narnia" posted:

Okay, call me stupid and for the record I don't listen to rap music but why is this so funny. It was a little funny but I really don't get this for the most part.

And Malchickiwick, some really clever persona who is my new best friend responded:

The style of the rapping is "gansta rap" which is generally performed by muscular, rich, hip, black artists, and the subject of the songs is usually "thug life," which involves drugs, womanzing, being hip, and having a lot of money.

In this case, however, the rappers are two, white Jewy guys with unimpressive physical attributes and little money or fashion sense.

Their song is about watching a geeky fantasy movie for kids, cutting corners on prices, and other unhip or mundane activities.

Thus, the form of the song is the opposite of the song's content, creating irony. Irony is one way of achieving humor.