Imitation may be the most sincere form of flattery, but to mock is to make fun of or mimic someone with contempt, ridicule or derision. "Louise’s favorite pastime was to mock her brother’s inability to sing on key."
The verb mock — as in the use that inspired the name of the mockingbird — can be simple imitation but, more often, to mock someone is to show disdain through mimicry or parody. As an adjective, mock indicates a fake or a simulation: "The design team made the most fabulous mock Eiffel Tower for our staging of An American in Paris." "His mock gratitude was, at times, unbearable."