I have repeatedly heard that if the Noble Messenger (PBUH) is ridiculed, this is considered an indication of a forthcoming triumph for Muslims. Is there any evidence for this?
All we have on this matter is mere implications, not decisive Shar`i texts. Scholars can act upon implications only if they do not clearly contradict with the apparent denotation of the texts. One such implication is found in the Qur’anic verse: {Truly! We will protect you against the scoffers} [Al-Hijr: 95]. Those were people who mocked the Prophet (peace be upon him), and he invoked curses on them. Thereupon, Allah destroyed them entirely.
Another example is: {Indeed, it is the one who hates you (O Muhammad) that will be cut off} [Al-Kawthar: 3]. This verse talks about a person who hated and antagonized Prophet Muhammad, so Allah deprived him of blessings and descendants.
A third example is: {So, Allah will protect you against them, and He is the All-Hearer, the All-Knower} [Al-Baqarah: 137].
All such verses imply that those who mock the Prophet will be doomed and that he is protected against whatever disturbs him. After all, these remain just inferential evidence, not express statements.
Anyway, we already see how such incidents of ridiculing the Prophet have caused Muslims everywhere to unite and consolidate, which is a sign of the Blessings conferred by Allah upon His Messenger. We ask Allah, the Almighty, to crown our unity and zeal with an outstanding victory.