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.