I teach in China, and recently gave my students a passage to parse about the Mandate of Heaven, and none of them (high school students) had heard of it. I asked more friends, ranging from 18-30, and almost none had heard of it. When I went to translate it into Chinese, my dictionary actually had an example sentence of "大破天命观", which translates to "eradicate the concept of the mandate of heaven". What was the actual process used to eradicate this concept?
Can you clarify a little bit? The concept of Tiānmìng 天命 is still absolutely taught in history classes here. What was the context that you were bringing it up in? What are the backgrounds of the people you're asking? I'm shocked to hear that you found so few people who had heard of it, so I'm interested in knowing a bit more about the context of your asking.
As an additional point, I'd be careful about giving Mao too much credit for this. Tiānmìng as a concept gained and lost importance over multiple time periods, and was often employed by official historians of new dynasties to support the new rulers legitimacy. There was no new state historian of a post-Qing dynasty to present it as such. A better question would be "For what reason did the concept of the Mandate of Heaven fall out of use". Otherwise you'd have to present some evidence that it was in fact Mao, and not apathy or entropy with no modern use for the concept. Remember the Qing did last for over two and a half centuries.
If you could present something in support of why you think Mao is responsible instead of some other cause, I'd be interested to hear it.
So to recap:
What is the context that you're asking in? What are the backgrounds of the people you're asking (e.g. wealthy Shanghainese vs impoverished Anhui farm kids)?
What is the reason you believe Mao to be singly responsible for it falling out of style?