Plath's poem presents the reality of aging. The mirror is our narrator; it expresses its ultimate honesty, explaining that it cannot lie or be mistrusted--it is "silver and exact" with "no preconceptions." The mirror is being personified here as something that could make judgments. It calls itself a "little god" with one eye which is "unmisted by love or or dislike."
Interestingly, it is the first stanza that gives the reader the sense that the mirror is praising itself--in effect, acting as its own mirror, reflecting the opinion it has of itself as an all-seeing being who "swallows" everything "just as it is." The opinion of itself as a god is the clearest indicator of the mirror's own high regard.
Despite the fact that it claims a lack of judgment, it comments on the pink speckled wall across from which it is placed and therefore must "meditate on." The mirror states,
"I have looked at it so long
I think it is a part of my heart. But it flickers.
Faces and darkness separate us over and over."
This in itself seems a sort of judgment, a weary or despairing--or perhaps even hateful--comment about the distance placed between the "part of my heart" that the mirror so often sees. The things distancing the mirror--faces and darkness, the flickering shadows--are then the things that the mirror allegedly fairly reflects.
Knowing as we do that mirrors must reflect what they see, and couldn't possibly hold judgment as inanimate objects becomes irrelevant when our narrator is a mirror. Plath implies the mirror is invested with feelings. Following this commentary about how truthful it is, though its heart is wrenched apart from it so often, it then gives a cold account of how it "sees" the woman.
The mirror appears to indeed make judgment in the second stanza, but softens due to the regular nature of the woman's appearances. Rather than the darkness perpetuating, the woman is the one to break it every day. This in turn seems to comfort the mirror, otherwise it would not comment on the darkness at all, implying a hint of loneliness. As the woman ages, according to the mirror, she becomes agonized: "She rewards me with tears and an agitation of hands." This seems a cruel comment--the mirror considering it rewarding to receive such a pained reaction for telling the truth.
This is where the analysis can come to the woman herself. If the mirror is an object of reflection, than the deeper idea is that the woman is also lonely, and the mirror must reflect that. Not only does this mirror reflect the physical, then--the woman is aging and has, in effect, "drowned" her younger self in the mirror--but also the emotional. The loneliness is prevalent in both characters. Furthermore, the mirror seems to be suggesting that the woman has wasted her youth waiting to become old, standing in front of the mirror each morning and wringing her hands at each new imperfection--each view of herself more and more a "terrible fish" in the reflective lake of the mirror.
Finally, I think it is important to note that mirrors have been known to lie as well. Funhouse mirrors, dressing room mirrors--each of these are altered to provide a certain impression. The further comparison of the mirror as a lake adds the finishing touch--lakes seldom reflect a true image, perhaps due to what is truly beneath.