Is saying jokingly to someone: "You want to prevent me from looking after you yourself have looked? Where are you with regard to the Hadith: 'None of you believes until he loves for his brother what he loves for himself'?" considered apostatizing mockery?

The statement mentioned is serious and it is not permissible to apply a hadith out of its proper context. However, it is not considered mockery of religion unless it is said dismissively or contemptuously. It is obligatory to be cautious of slips of the tongue, for the Prophet, peace and blessings be upon him, said: "Indeed, a servant may utter a word, not realizing its implications, by which he will fall into the Hellfire farther than the distance between the East and the West." Narrated by Muslim. That word does not necessarily have to be a word of disbelief; rather, it could be a word that results in harm to a Muslim or similar, and this is an encouragement to guard one's tongue.