Is this considered mockery of religion and apostasy, given that a clip was found of a man in the desert saying to another: "O youth, shall I tell you of three things which, if you do them, people will love you?" and someone commented that these words are taken from the beginning of a Hadith of the Prophet, peace and blessings be upon him?

There is no mockery in the words of the two individuals; an elder may advise a younger person by saying: "O boy," as the Prophet, peace and blessings be upon him, said to Ibn Abbas: "O boy, I shall teach you some words: Be mindful of Allah, and Allah will protect you..." This quote from the Prophetic sayings is permissible if used in a praiseworthy context.