Is imitating a person's voice mockingly, while uttering the phrase "the Prophetic Sunnah," without intending to mock the Sunnah, considered mockery of it?

What happened to you is not considered mockery because you did not intend it. You must ignore it and turn away from it. It seems that you are suffering from waswasah (whispers from Satan) regarding the matter of mockery. Therefore, you must turn away from these delusions and not allow Satan to gain power over you.