Most of Diamond Tiara's appearances involve her making fun of others and acting superior. In Call of the Cutie, Diamond Tiara makes her first appearance alongside Silver Spoon, inviting Apple Bloom and Twist to her cute-ceañera, a party for those who have recently earned their cutie marks. (The name is a pun on the Spanish quinceañera, a girl's "sweet 15" or coming-of-age party when a girl turns 15.) It's heavily clear that they were only invited based on their lack of cutie marks, with the intention that they be mocked for Diamond Tiara's enjoyment at the celebration.
"I mean, waiting for your cutie mark is sooo last week. You got yours, I just got mine. We all have them already. [gasp] I mean, almost all of us have them already."
— Diamond Tiara
Twist gains her cutie mark before the party, but Apple Bloom does not. At the cute-ceañera , Apple Bloom tries to hide her lack of cutie mark with a tablecloth, but bumps into Diamond Tiara and Silver Spoon, claiming that she got her cutie mark earlier but doesn't want to show off and take attention away from the party's guest of honor. Diamond Tiara replies that she didn't want to see Apple Bloom's cutie mark anyway. As Apple Bloom prepares to leave, she trips over the long tablecloth and hits her head on the table that a phonograph is stationed on, thus exposing her blank flank. Diamond Tiara and Silver Spoon see that she has no cutie mark and don't hesitate to make fun of Apple Bloom, who is soon joined by two other blank flanks, Scootaloo and Sweetie Belle. The teasing gets ignored as Twilight Sparkle and Applejack explain that being a blank flank means having not yet discovered one's true potential. Diamond Tiara is later seen standing alongside Silver Spoon in the stairway, watching the other ponies play party games while upset at the loss of the attention she was receiving.
She appears again along with Silver Spoon on stage at the talent show in The Show Stoppers, as well as during the lesson in The Return of Harmony Part 1 and in The Cutie Pox with Diamond Tiara trying to prove in the school yard that Apple Bloom's cutie mark is a fake. Later she asks Cheerilee if it's really fake and is seen being jealous of Apple Bloom's loopty-hooping ability.
She appears in Family Appreciation Day, alongside her father, Filthy Rich. When she visits the farm with her father, she points out Granny Smith's odd behavior to Apple Bloom and convinces her of how humiliating it would be if Granny Smith spoke to her class. After Granny Smith impresses Apple Bloom's class with her presentation of how she helped found Ponyville when she discovered and learned how to cultivate zap apples, Diamond Tiara openly denigrates her. At the end of the episode, her father, evidently furious that his daughter insulted the most valuable produce supplier for his business, forces her to participate in singing to the water cans, much to her frustration.
In her next appearance in Ponyville Confidential, she is the chief in charge of the school paper: the Foal Free Press. She appoints the Cutie Mark Crusaders the writers of the gossip column to try to revive the newspaper. When the Cutie Mark Crusaders attempt to resign their position, Diamond Tiara blackmails them with embarrassing photos. Diamond Tiara is later stripped of her position by Cheerilee because of her malpractices, being replaced by Featherweight. In the end of the episode, she is demoted to take care of the printing press and the previous holder of that position, Shady Daze, becomes the photographer and decides to take a picture of the demoted Diamond Tiara. The flash from the camera dazes her and makes her stumble upon the printing press, making her dirty with ink. As everyone is laughing at Diamond Tiara, the episode closes with her looking bitterly at the audience.