Question

why does the woman behaves like a terrible fish in the poem mirror

Answer 1

The mirror does describe this poem, and so it is personified this way.

However, the mirror compares to itself to the lake, saying "I am a lake."

This is just an example of a metaphor.

Mirror elaborates in the final line that "in me, she has drowned a young girl and now she is growing old day by day like a terrible fish."