Is it permissible to laugh or smile when reciting Quranic verses that deal with stories of fools and disbelievers and Allah's mockery of them, or to rejoice at the triumph of truth and be pleased with the circumstances of the victorious believers? And is that considered a lack of decorum or a lack of humility, especially if it is during prayer?

There is no harm in smiling or laughing outside of prayer, rejoicing in Allah's victory and the destruction of His enemies, or in divine blessings, as Allah Almighty says: "And on that day the believers will rejoice" (Quran 30:4) and "Say, 'In the bounty of Allah and in His mercy - in that let them rejoice.'" (Quran 10:58). The Prophet, peace and blessings be upon him, smiled when Surat Al-Kawthar was revealed, when he saw a group of his community sailing the sea, and during ablution (wudu') which expiates sins. As for smiling during prayer, it is disliked, and loud laughter invalidates it.