The answer is:
The mood is lighthearted because the comparison created is silly.
A simile is an explicit comparison between two different things to make a description more resounding. It usually uses words such as like, as, than, and verbs similar to resemble.
In the lines from the poem "My Cat," the narrator creates a cheerful mood in a simple analogy: the movement of the cat on the pillow and the movement of pudding on a plate.