The title, "Mirror," tells the reader who the speaker is in this poem, though we probably could have figured it out without the hint. The title keeps the poem from being gimmicky. We don't spend the whole poem trying to figure out who the speaker is like in a riddle, but we're still a little surprised, or at least intrigued, when we find out that the poem is not merely about, but spoken from the point of view of, a mirror.
In addition to letting the reader know the identity of the speaker, the title "Mirror" has something to say about this poem, and poems in general. All poems, in a way, are mirrors – we look into them, and what we get back from them is, in a way, a reflection of ourselves. What kind of reflection does this poem show you?