"And the mome raths outgrabe."
The repeated short "a" sound in "mome," "raths," and "outgrabe" creates a sense of playful absurdity and emphasizes the nonsensical nature of the Jabberwocky's world.
This is just one example, and there are many other instances of assonance throughout the poem. Carroll uses assonance to create a sense of rhythm and musicality, which further enhances the fantastical and dreamlike quality of the poem.