Just like most epics with a central character, "Jabberwocky" is simply titled after the most significant thing in the poem – the giant monster foe. The title of this poem forces us to reckon with the monster as the central force of the poem. It shifts our attention to the monster, and away from our anonymous hero.
But why not the hero, à la Beowulf? Well, our hero doesn't have a name. (The why of this is a study question, so we'll leave it up to you to ponder Sir Hero's anonymity.)