Though it may seem unusual to some on first reading, Acevedo's decision to write The Poet X in verse is highly appropriate. The story's protagonist, Xiomara, is someone who lives for poetry. She doesn't just write it, she lives it. And so it seems perfectly reasonable for the book, which deals with her experiences of the world, to be written in verse.
Poetry is the primary means by which Xiomara understands the world around her. It is only through words that her bewildering array of life experiences makes any kind of sense. In particular, Xio uses poetry as a kind of therapy to help her deal with the many conflicts that she encounters, such as the conflict with her parents over dating. Writing about these conflicts in verse allows Xio to say something important about them in a way that would never be possible in prose.
For Xio, language is primarily an instrument, a means of self-expression, and as poetry occupies a very special place in her soul, it's only natural that she should express her thoughts and feelings through the medium of verse. A soul steeped in poetry such as hers can only really reveal itself and its inexhaustible depths through the poetic language in which The Poet X is written.