What is the contrast between Puck and Bottom?
Bottom is a mortal who is a control freak and basically intolerable. His name says it all, and later in the play when his head is turned into that of an "ass" it culminates his attitude and personality.
Puck is a Pixie--not a mortal. He is not intolerable, he just enjoys playing pranks. He serves Oberon the King of the Pixies with loyalty and zealous joy.