The Sky is Everywhere almost has two endings: a prose ending and a final poem. The prose ending shows Lennie leaving Gram's letters to Paige at Bailey's gravesite, then hurling the Lennie plant over the edge of a cliff because it no longer represents her. To fully appreciate all that Lennie's letting go of, be sure to read up on the Lennie plant over in the "Symbols" section—here, we'll just say that she's finally come into her own and is ready to move forward with her life.
The poem ending, though, is really different. It shows the love poem that Lennie wrote for Joe and then got really embarrassed about. The words are mostly unreadable because Lennie crossed out every line, and the note under the poem documents all the times Joe attempted to preserve it (aw) after Lennie tried to ruin it. So contrary to the prose ending, this ending is all about Lennie coming together with Joe instead of Lennie coming together with herself.
These different endings also have different tones. Lennie is sad at Bailey's grave, but by letting go of the plant, she accepts that she has changed. So the tone of the prose ending is a resigned sadness, whereas the lover's spat about the poem is playful.
In a way, the contradictory endings mirror the larger book and it's dual stories about grief and falling in love. So while the tones of the two endings don't quite match, they still happen at the same time, side-by-side, coexisting just like grief and joy have for much of the story.