Growing Pains
Much of the dramatic arc in the story comes from the tension between Greg's tendencies towards being immature and the pull to grow up and assume more responsibility. We see this play out when Greg's mom takes him out to ice cream to celebrate doing the right thing, even though Greg has let his best friend take the fall for his own misdeeds on Safety Patrol. On the other hand, we see that to the adult world, Greg and his friends are still children, with their video game intake monitored by adults and the constant threat of being grounded.
Sibling Rivalry
One of the hardest parts of being a pre-teen is dealing with your siblings, and Greg really has a handful. We know that he is tormented by his older brother Rodrick, but Greg also seems to look up to him, given his infatuation with listening to Rodrick's heavy metal CDs and the admiration he has for how Rodrick has written all over an old yearbook. Greg is also quite jealous of his little brother Manny for getting so much attention and always getting a free pass on making a mess. As the middle brother, Greg really does seem to feel like he's stuck in the middle.
Friendship
There's never quite a point in the book where Greg describes someone as a friend excitedly. If anything, we only see Greg talk about friends when he's trying to gain leverage over someone else, such as talking about Rowley as a friend to justify feeling mad at Rowley for abandoning him. On the other hand, if we focus on Rowley's character, we see a more simple and innocent embrace of friendship, as Rowley is always laughing at Greg's jokes and gladly goes along with whatever games Greg dreams up.
Popularity
In contrast to the theme of friendship, we also have the theme of popularity. Just like how friendship comes more simply to Rowley, it ends up that popularity comes more simply to him too, as the big twist at the end is that Rowley wins the coveted Class Clown title in that grade's yearbook. On the other hand, Greg obsesses over popularity and attracting the attention of his classmates. Greg's obsession is a sign of the insecurities that naturally come with the confusing time that is middle school, and Greg's struggles with popularity also tie into other early teenage concerns and types of validation, such as being liked (via his desire to write the school comic), his body image (his desire to lift weights), and winning the attention of girls.
Doing the Right Thing
This theme is stated clearly when Greg's mom advises him to do the right thing when Greg is troubled over an incident about which she doesn't know all the details. But we see it come up a lot. Does Greg do the right thing when it comes time to write thank you cards for Christmas presents, or does he do the easy thing? Does Greg join the Safety Patrol because he wants to help, or is it just a ploy to get out of class? Ultimately, Greg does do the right thing at the end of the book when he says he threw the cheese on the basketball court, saving Rowley the embarrassment of admitting that he was forced to eat the cheese. It results in Greg losing some popularity, but he saves face for his buddy and makes up for having let Rowley take the fall for his own mistreatment of the kindergarteners.
Authority Figures
A lot of the humor in this book comes from Jeff Kinney portraying a series of bizarre adults through the eyes of a middle-school boy, and ultimately we get a complex idea of how kids think about authority figures. On one hand, we have Mr. Winsky, who has the ability to punish and disgrace both Rowley and Greg, but on the other, we have Mr. Ira who uses Greg's comics to boost his own education agenda. We get the impression that adults have power—like Greg's parents—but that they use that power for silly, even petty ends, such as Mrs. Norton adding a seemingly endless amount of new characters and songs to The Wizard of Oz.
Video Games
Jeff Kinney has said in interviews that before he wrote Diary of a Wimpy Kid, he wasted a lot of time playing video games, and he portrays his protagonist as obsessed with video games too. Video games are a way for Greg and Rowley to build a friendship, and a stand-in for Greg's disappointments with growing up, as he no longer gets all the fun toys like his brother Manny does and starts to get more adult gifts. Instead of giving him the video game Twisted Wizard, his mom accidentally gives him a sweater she meant to give to someone else.
