Is the saying about poetry, "like thunder," considered a mockery of religion, knowing that thunder is an angel?

Thunder is an angel who possesses fiery prods with which he directs the clouds wherever Allah wills. His voice is his chiding of the clouds. This is the view of the majority of scholars. Sheikh Al-Islam Ibn Taymiyyah said that thunder is a sound that chides the clouds, and lightning is the gleam of fire in the angel's hand. As for your sister's statement, "Your hair is like thunder," it is a simile comparing thick, spreading hair to thunder (meaning a spreading sound), and it has no connection to mocking religion. One must beware of the whispers of Satan.