Interestingly enough some of the earliest critiques of Mao's Cultural revolution came from Mao himself in 1975.

The Chairman told Mao Yuanxin to prepare for a gathering of Politburo members to consider specifically what in their view ought to be the verdict on the Cultural Revolution. Mao's aim was to have a meeting of the Politburo, chaired by Deng, arrive at a consensus in support of his own verdict that it had been 70 percent success, 30 percent failure. Mao also wished to see a formal resolution to that effect drafted and passed. - Roderick MacFarquhar, Mao's Last Revolution

This 70/30 line was later adapted by Deng Xiaoping after Mao's death for his entire period in power, so interestingly enough Mao himself decided on the percentages of his success and failures.

To truly understand the criticism for Mao after his death we have first consider the people most likely to be impacted by the Cultural Revolution. The targets were mostly intellectuals and other people accused of adhering to the "Four Olds"; Old Customs, Old Culture, Old Habits, and Old Ideas. In practice this could basically be anyone, but being educated certainly wasn't a mark in your favour to the red guards. As an uneducated peasant you were most likely to survive the period mostly unharmed.

So we can conclude that the Cultural Revolution was mainly targeted against the Maoist China equivalent of the working and middle classes, which didn't do much for Mao's popularity in the end.

An added effect of the Cultural revolution was the devastation of established power structures, i.e. local party-organisations and the local bureaucracies, which greatly reduced their power and ironically made it easier for the reformers to ignore their wishes and press on with economic liberalisation.

On 6 October 1976, four months after Mao's death, the so-called "Gang of Four" was arrested. These were people close to Mao who were active during the cultural revolution, and after Mao's death they lost his protection against the many enemies they had made during the Cultural Revolution. It was decided they would be convenient scapegoats to blame for the excesses of said cultural revolution, conveniently redirecting some of the blame that should really have gone to the address of Mao, thereby making criticizing the gang of four an easy way for the people to actually criticize the policies of Mao.

To understand why the Chinese leadership did this, we also have to look at the position of Mao in the history of the Chinese Communist Party: they derived their legitimacy from his early efforts, and they didn't have a Lenin to fall back on after denouncing him like the Russians did with Stalin in the Soviet Union. Criticizing Mao too directly would be tantamount to criticizing the party and understandably this was not something the Party was too keen on. Nevertheless, in the Resolution on certain questions in the history of our party since the founding of the People’s Republic of China, the party declared the following:

19. The “cultural revolution", which lasted from May 1966 to October 1976, was responsible for the most severe setback and the heaviest losses suffered by the’ Party, the state and the people since the founding of the People’s Republic. It was initiated and led by Comrade Mao Zedong. https://www.marxists.org/subject/china/documents/cpc/history/01.htm

This criticism, both from inside and outside of the Party, made pure ideological Maoism lose its appeal, especially seeing the rough period of the Cultural Revolution the country had just been through. This paved the way for the pragmatism of later leaders such as Deng Xiaoping, whose maxim "It doesn't matter whether a cat is white or black, as long as it catches mice it is a good cat." nicely encapsulates said pragmatism. Ironically the entire reason Mao started the Cultural Revolution was to prevent revisionists from taking over after his death like Khrushchev had done with Stalin, but the counterreaction to the cultural revolution was in fact a boon to the revisionists elements in the party.

Anyway, let's go back in history a bit to the matter of Mao's succession. Deng Xiaoping was the most influential reformer of the post-Mao period, and his machinations within the party saw the radical Maoists within the party deposed and stripped of their powers. An example would be Mao's chosen successor, Chairman Hua Guofeng, who promised after Mao's death We will resolutely uphold whatever policy decisions Chairman Mao made, and unswervingly follow whatever instructions Chairman Mao gave", which mockingly was called the "Two Whatevers" by the followers of Deng.

Now, on to the public. In 1978 a Chinese artist started putting his poems on a wall in Beijing. This proved to be popular and this "Democracy Wall" was quickly covered by pamphlets criticizing the Gang of Four and Hua Guofeng. Because this suited Deng he let it happen until the people started criticizing Deng as well, after which he moved the wall to some small building in a park for outside of the city centre. A year after that there was a constitutional amendment that removed the freedom to put pamphlets on walls, ending the wall's short but illustrious life.

Eventually in 1980 Deng managed to replace Hua Guofeng as premier with one of his own people, the reformer Zhao Ziyang, giving him free reign to reform the economy. In 1981 he managed to take away Hua's other titles, Chairman of the Party and his seat on the Central Military Commission, permanently taking away the last hope for a return to radical Maoism.

Deng and Zhao then ended the collectivization of the peasants, giving them the right to sell some of their produce themselves and allowing them to pursue employment on the side of their agricultural works. This opened the floodgates to further economical reform, seeing as how this seemingly simply policy increased agricultural yields by a significant margin.

Thus ends the era of Maoism in China. Any questions?

Sources: Archie Brown, The Rise and Fall of Communism Roderick MacFarquhar and Michael Schoenhals, Mao's Last Revolution

Small Edit: Jesus christ this took a long time, was fun to page through my books to search for the answer though. I can seriously recommend Archie Brown's book if you seek further accessible information for reforms in Communists countries.