It might seem that a poem written from the point of view of a mirror would have a pretty boring speaker, but that's not true of this poem. This mirror has a lot of human-like qualities, which keep us interested. In addition, the speaker's voice remains the same throughout the poem, although it changes in form between the first and second stanza from a mirror to a lake.
We can see the mirror clearly in the first stanza: it's silver, four-cornered, and situated across from a pink speckled wall. We can imagine the mirror hanging in a woman's bathroom, perhaps remaining there for years as different women move in and out of the house; we don't hear too much about the humans in the first stanza except that their faces separate the mirror from its beloved pink wall. We'd think a pink speckled wall would be a little suffocating to look at for the majority of your life, but the mirror feels as if the wall is a part of its own heart. This mirror is also proud of its reflecting skills, calling itself the "eye of a little god," reflecting truthfully exactly what it sees. We could imagine the mirror as an artist in the school of realism, bragging about how true to life it is.
When the mirror switches into a lake, it holds onto these same characteristics, calling less exact reflections, like those influenced by the moon or candles, liars. It claims to be important to the woman it reflects, and she seems to be important to the lake – it reflects her back "faithfully" and seems upset when it sarcastically says she "rewards" it with tears. Yet this women isn't elevated to the status of the pink wall, which the mirror thinks is a part of its heart. In a poem spoken by an inanimate object, we can expect some partiality to other inanimate objects, right?
While we still get a clear picture of the characteristics of the lake, unlike we did with the mirror, we don't really know what the lake looks like, though it must be still enough to show a reflection. We imagine this is a small lake, within a morning's walk from where the woman in the poem lives – close enough that she can peer at her reflection there every morning.