It seems to me that many American books, short stories, and poems have the theme that knowledge is not important if you are not imaginative. For example, Joy in “Good Country People” thinks she is smarter than everyone because she is intellectual and educated, but she is not imaginative enough. I need a 21st-century American poem or short story that emphasizes this theme.

I think you have picked an excellent theme that describes a lot of contemporary American literature. To help you select a text that best suits your tastes and needs, I will discuss several options that address the theme of innovation.
For poetry, I would suggest the poem “Introduction to Poetry” by Billy Collins. This poem is written from the perspective of a teacher who has difficulty getting students to understand or appreciate poetry. It fits very well with your explanation of theme because the student’s lack the imagination required to truly study poems.
For a novel, you might consider The Fault in Our Stars by John Green. This novel fits with your theme because the protagonist, Hazel, only begins to see beyond her myopic, logical view of life after falling in love with a highly imaginative young man.
For a short story, you should consider exploring the works of Chuck Palahnuik, George Saunders, and Junot Diaz. Although I can’t pinpoint one specific story to use, each of these authors is known for his imaginative style.
I hope this helps you select the best work for your analysis. Good luck!