One of the central themes of "The Metamorphosis" is explored in the reaction of Gregor's family to his fate. Gradually, bit by bit, they cease to view him as a human and come to regard him as an obstacle to their own happiness, and as a bug that must be killed and thrown away into the garbage so that they can live their own lives in peace. This is clearly shown through the ending of the story, when, in spite of just having lost their brother, the mother, father and sister all seem to enjoy a new beginning that is notable in the hope and happiness that it gives them all now that Gregor is dead and out of the way:

The streetcar, where they were the only passengers, was flooded with warm sunshine. Leaning back comfortably in their seats, they discussed their future prospects and concluded that, upon closer perusal, these were anything but bad; for while they had never actually asked one another for any details, their jobs were all exceedingly advantageous and promising.

Kafka seems to be stressing the way that even the closest of family relationships can change when external factors alter certain aspects of society. Additionally, given the way that Gregor's life can be viewed as not changing that much from before his metamorphosis to after his change, Kafka could be painting a very dark picture of human nature and the way that characters that are so important one day can be forgotten the next.

In "Mirror," by Sylvia Plath, the theme is that of aging and one woman's inability to accept how inevitable the process is. Note how this theme is highlighted in the last three lines of the poem:

Each morning it is her face that replaces the darkness.
In me she has drowned a young girl, and in me an old woman
Rises toward her day after day, like a terrible fish.

The vivid and somewhat terrifying final image of a "terrible fish" that rises towards the woman captures both the inevitability of the aging process and the woman's feeling of horror as she has to confront her own wrinkles and signs of physical deterioration. The theme of this poem is clearly different to that of "The Metamorphosis," although both texts focus on a change and the impact of that change on those concerned.

What thematic similarities exist between Kafka's The Metamorphosis and Plath's "Mirror"?

One major similarity that you could build on for a thesis statement is that both "Metamorphosis" and "Mirror" deal with the theme of transformation as well as the theme of identity.

Plath's poem "Mirror" conveys the transformation of self, from young to old:

"In me she has drowned a young girl, and in me an old woman
Rises toward her day after day."

Plath uses extended metaphor to compare the mirror to a lake, and the woman in the mirror has lost the original image of herself as a young girl in the mirror; as she ages, she sees her transformation into an old woman.  The mirror becomes an agent of revealing transformation.

Moreover, Kafka's character Gregor also experiences transformation; his form shifts into that of a terrible insect.  His transformation raises important questions of identity and what does it mean to to be human. 

Both "Metamorphosis" and "Mirror" challenge the reader to discern what role the characters' transformations have on their identity, of who they are as a person, and their own humanity.