What is the ruling on mocking one of the customs or rituals of the non-Muslims even though one fears that this may have been a custom sent down by Allaah to earlier specific generations? One such custom may be putting stickers on one's head, which Hindus do. I thought that this might be a custom sent by Allaah to a specific generation through an earlier Prophet, but I still mocked it just because I wanted to. I did not have to do it. I hope you can give me an answer.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

It is unbecoming of the Muslim to mock people. A wise believer refrains from such an act. Time is a great blessing; the Muslim's time is too precious to be wasted in such an action that brings no benefit for him or for Islam; rather, it may incur harm on him. Ibn ‘Abbaas, may Allah be pleased with him, reported that the Prophet, sallallahu ‘alayhi wa sallam, said, "Two blessings which many people squander: Good health and free time." [Al-Bukhari]

Instead of mocking people, the Muslim should be concerned with their situation and try to guide them to the truth so that he would earn the great rewards for that. Sahl ibn Sa‘d, may Allah be pleased with him, reported that the Prophet, sallallahu ‘alayhi wa sallam, said to ‘Ali, may Allah be pleased with him, on the Day of Khaybar, "Then invite them to Islam and inform them of what is obligatory on them from the Rights of Allah, for, by Allah, if Allah guides even one person through you, that is better for you than possessing the most precious red camels." [Al-Bukhari and Muslim]

Your assumption that the Hindu customs might have been revealed by Allah to a past generation through an earlier prophet is wrong and has no valid grounds. Hinduism is a pagan religion of disbelief that has nothing to do with the divine religions.

Allah knows best.