What is the effect of Plath's choice to write the poem from a mirror's perspective?

It is startling and disorienting to have this poem written from the point of view of a mirror. A person looks into a mirror to receive reassurance or confirmation about their own appearance or reality. We expect that if we see something amiss in our appearance in a mirror, we can quickly fix it. We expect a mirror to provide a reflection of ourselves and nothing more. Beyond its purpose in reflecting us, a mirror seems empty.
It is startling, therefore, to think a mirror could look back at us, see us, and judge us, not merely reflect us. It is disorienting that we might not only see ourselves but that the mirror itself might have a consciousness that could see us clearly and objectively.
A mirror is an object women look into both to see themselves and, in the second stanza, to try to see into their own souls—to see their depths, as in a lake. However, Plath, by personifying the mirror and giving it a personality, a consciousness, and a perspective, seems to be placing the mirror in the role traditionally held by women vis-a-vis men. In other words, men look at women for reassurance and confirmation that they are who they think they are. Men, Plath implies, have no realization that women have a consciousness that sees them just as they are, unvarnished—just as how most people don't assume mirrors have consciousnesses.
But complicating this is the fact the mirror is looked into by a woman. The mirror realizes that it is important to the woman, and it helps her to replace her girlish self-concept with the more terrible concept of looking older.

Plath's poem "Mirror" is written from the perspective of an anthropomorphized mirror, which has "no preconceptions." The effect of writing from the perspective of this inanimate object, which is "not cruel, only truthful," is to create the impression that the narrative is completely objective. Rather than allowing the woman who "bends over" the mirror to describe herself, bringing her own preconceptions and feelings to bear, the mirror can only reflect "faithfully" what it sees.
Offering an insight into the mirror's limitations, furthermore, evokes a sense of pathos in the reader as we see the futility in the woman's search "for what she really is." The mirror is not like "those liars, the candles or the moon," who can alter what the woman sees. Instead, the image of the woman in the mirror is "unmisted by like or dislike."
The emphasis upon the mirror's truthfulness is reiterated throughout the poem: the mirror is faithful, "truthful," "the eye of a little god." The mirror is all-powerful in that it can show everything exactly as it is. Given this, then, the final line of the poem acquires an added power: the old woman who "rises . . . like a terrible fish" cannot be a figment of the woman's imagination or a product of her fear, but simply a truth which the mirror must slowly reveal.