Primadonna literally translates to "first lady" and refers to the principal female singer in an opera company. However, it's often used metaphorically to describe a person, especially a woman, who is vain, temperamental, and demands to be the center of attention.
In this song, the singer uses it sarcastically to call out a man who is self-absorbed and acts like he's the most important person in the world, flipping the traditional gender association of the term.
Have you ever met someone so self-absorbed they think they're the center of the universe? In "PRIMADONNA," Clara sings about her frustration with exactly this type of person. She calls herself "stupid" for even talking to a guy she dislikes, a man who is obsessed with his own reflection and only ever talks about himself. The lyrics paint a picture of someone who is all talk and ego, convinced of his own importance.
Clara declares that she's finally had enough. Starting tonight, she's going her own way and cutting him out of her life completely. The most powerful twist comes when she flips the script. While he might call her names, she points out that he is the real "primadonna"—the diva who demands all the attention. She ends the song by wondering why she ever made such a "big mess" over such a "small, small, small, small man."