Based on the language of "this is just to say," the poem could serve as a

Based on the language of "This Is Just to Say," the poem could serve as a casual note left on the kitchen table.
The poem is quite short and tells us a story about how the narrator ate the plums which were in the icebox. He asks that person for forgiveness if she wanted to eat those plums first, but he had to eat them because they were delicious. That's everything there is to the poem, which is why it can only resemble a casual note, and not anything else.