Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
The laughter of a dead person in a dream indicates their good state in the Hereafter and that they are blessed.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
Whoever sees a dead person laughing, goodness will reach him from them, such as prayers or charity.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
The laughter of the dead is glad tidings for the dreamer of the righteousness of his state and his good end.