Is saying mockingly, "Don't sit there and tell me: 'The most enduring, even if little; the most enduring, even if little,'" upon hearing the Hadith: "The most beloved of deeds to Allah are the most enduring, even if little," considered an act of disbelief (kufr)?

A servant must balance their actions. You did not intend to mock the Hadith (definition), but rather wanted to illustrate the existence of other texts that encourage diligence, and this does not warrant your excommunication. Allah Almighty says: "But whoever opens his breast to disbelief - upon them is wrath from Allah, and for them is a great punishment. That is because they preferred the worldly life over the Hereafter and that Allah does not guide the disbelieving people." You must show good manners when dealing with the Sunnah (definition) and choose the best words when speaking about it, for "And whoever honors the symbols of Allah - indeed, it is from the piety of hearts." Ibn Umar severely rebuked his son when he objected to a Prophetic Hadith, and did not excuse him for his ill-manners towards the Sunnah.