What is the central idea of the poem "Mirror" by Sylvia Plath?

“Mirror” is an objective perspective on time and mortality
and particularly about beauty – or the ephemeral nature of
beauty and the superficiality of beauty.
Mirrors are unconscious, so they can’t be harsh. They are only truthful; unbiased in
their reflection of the world. Nor do mirrors interfere in the world. The mirror then
describes itself as a lake, with the woman looking older each time she resurfaces to
reflection. The mirror says the woman drowned a young girl (the woman as a girl) in
itself (the mirror). The mirror notes it is important to the woman. The woman needs the
mirror as she defines herself according to her own beauty; perhaps trying to live up to
superficial standards of her social world. The mirror says the woman searches “my
reaches for what she really is.” The mirror is an unconscious thing, but as personified,
the mirror can only ‘wonder’ who the woman really is. As far as the mirror ‘knows’
(again, personification), the woman is obsessed with image (her own). But if the woman
is searching the mirror’s reaches for what she is beyond her superficial beauty, it
seems the woman can’t get that far because she ‘rewards’ the mirror with tears. So,
perhaps the implication is that the old woman resurfaces like a ‘terrible fish’ not
because she is old and lost her beauty but because she only values herself for beauty.
She is actually doomed to fail because you can’t probe a mirror for more profound
significance of the meaning of her life. A mirror is, by its physical nature, a
superficial reflection.