The title of this poem hasn't always been the same thing—at one point, this poem was apparently published as "Metaphors for A Pregnant Woman", which is a bit on the nose if you ask Shmoop. We think that title takes all the fun out of reading the poem, and makes the whole riddle thing pretty moot. We highly approve of Plath's change to plain and simple "Metaphors."
That said, the title "Metaphors" doesn't give us much to work with when we're trying to suss out what's going down in the poem. But it does tip us off that this poem is full of—say it with us—metaphors. Sure, that's an obvious point to make, but it's key in figuring out how to read the poem. It's saying one thing, but meaning another.
Plus, it's feasible to figure out that the metaphors are about pregnancy by the end of the poem without any help from the title, so it's not like we miss the cue all that much. Yet the title is important because it puts us on guard and, along with the first line, lets us know that we're supposed to be solving a riddle.