What is the ruling on someone who says jokingly and to gain sympathy from those around him: "I will jump off the prayer mat," intending to harm himself? Does this fall under apostasy by way of mockery?

The phrase "I will jump off the prayer rug" is not considered disbelief (kufr). The person suffering from obsessive thoughts (waswas) should ignore the whispers of Satan and occupy their time with learning, inviting people to Allah, seeking righteous company that helps them in obedience, or engaging in reading books and listening to religious lectures. This should be done with the conviction that disbelief does not occur through ambiguous words, especially for those suffering from waswas, because averting legal penalties from Muslims takes precedence over imposing them mistakenly.