From an earlier answer of mine

Part I

The view of these two imperial rulers held by both the Soviet state and its historical establishment was somewhat Janus-faced. On one hand, the state enshrined a Marxist-Leninist view of history that stressed the importance of large, impersonal forces and economic structures as the motive engine of historical development. Rulers, in such a schematic view of history, were byproducts of their time, and in the case of the tsars, beholden to the power of feudal and gentry social structures. However, starting in the Stalin period there was a marked tendency of the Soviet state to find a usable past and celebrate historical figures as precursors to the Soviet experiment. This recovery of a usable past frequently had to square this with existing Marxist methodology, which was not always the easiest of tasks. The status of Catherine II versus Peter I within Soviet historiography exemplified the difficulties in this process. While the earlier tsar emerged as a heroic figure who at times transcended his unique historical epoch, Catherine's image often suffered at the hands of Soviet writers.

The Soviet rehabilitation of Peter I started in earnest during the Stalinist period, but its roots stretched back into the 1920s. Much of the work produced on Peter I during the foundation of the Bolshevik state centered on the necessity of the tsar's use of force to modernize the state. For example, Lenin in his "Left wing" Childishness and the Petty-Bourgeois Mentality would argue that

our task is to hasten [the copying of German state capitalism] even more than Peter hastened the copying of Western culture by barbarian Russia, and we must not hesitate to use barbarian methods in fighting barbarism.

This approach to Peter I was not entirely uncritical. Mikhail Pokrovskii's two-volume survey of Russian history condemned Peter I's arbitrary power and saw it as emblematic of the gentry class's true face. But despite this acknowledgment of Peter I's "barbarian methods," the approach of historians in the 1920s cemented the idea that Peter I's reign was an important transformational period in Russian history, which begged comparison to both 1917 and the foundation of the USSR. The Russian Marxist theoretician Georgii Plekhanov, whose works would be reprinted during this period, concluded in one of his essays on Russian history:

The very character of Russian culture changes with appearance; our old, Asiatic economic way of life vanishes, ceding its place to the new way, he European one. The working class is destined to complete in our land Peter's great undertaking: to carry the process of Europeanization of Russia to its conclusion. But the working class will give a totally new character to this task, on which the very existence of Russia as a civilized country depends. Begun at one time from above, by the iron will of the most despotic of Russian despots, it will be finished from below, by the means of a liberation movement of the most revolutionary of all the classes ever known to history.

Plekhanov's sweeping verdict acknowledged the illiberality of Peter I, but connected it to his necessity for Russia to move forwards in its historical development. The 1930s saw the former element gradually erased in both historical materials and artistic presentations of Peter I and instead emphasized the fundamentally heroic nature of his transformation of Russia. The epic film Peter the First made the tsar a heroic protagonist and earned the personal approval of Stalin himself. Soviet pedagogy also emphasized Peter I as a workman-like figure not trapped by the class he was born into, but rather as someone who understood how to manipulate the social currents of his day to develop Russia. This view of Peter I sharpened the 1920s picture of the tsar; instead of painting him as a force for broad social change, the Stalinist historiography specifically emphasized his state-building and mastery of geopolitics as virtues to be emulated. Peter I's personal tyranny also became less of a moral issue and one made necessary by feudal opposition to his modernizing reforms. The post-Stalinist period preserved much of this heroic image of Peter I. The 1979 entry on Peter I in the Great Soviet Encyclopedia valorized his military and political skills and "despite all the contradictions in his character, he is known in Russian history as a progressive statesman and military figure."

Soviet historiography was not so generous towards Catherine II. Although Catherine II was as much of a state-builder and cultural reformer as Peter I, there was relatively little celebration of Catherine II in the USSR. Part of this was the continuation of pre-1917 critical historiographic traditions which tended to look at Catherine II's pretenses of being an enlightened monarch as mere window-dressing for her autocracy. But there were other problems that the German-born Catherine II presented to Soviet historiography and official memory that contributed to a negative picture of the monarch. While hagiographic history could portray Peter I's fondness for foreign innovations and ideas as necessary given Russia's "Asiatic"-level of development, celebrating this under Catherine II was problematic. As Stalin would explain in an interview with the filmmaker Sergei Eisenstein in 1947:

Peter I was also a great ruler, but he related to foreigners too liberally, openned the gate to wide to foreign influence and allowed the Germanification of the country. Catherine allowed even more. After that- was Alexander I's court really a Russian court? Was the court of Nicholas I really a Russian court? No, they were German courts.

Just as Peter I's modernization heralded the wholesale modernization of Russia which could be completed after 1917 Catherine II's court and rule turned the upper echelons of the state towards decadence and ultimate irrelevancy. Her German origins and Francophillia became even more problematic from the late 1930s as the Soviet state became more Russocentric in outlook and ethos as Great Russians became the "elder brother" in the Soviet family of nations. The Soviet musicologist B. V. Asaf'ev would lament in letters to the playwright Bulgakov that Catherine II's patronage of foreign arts inhibited the development of a truly Russian national arts. While Peter I was a ruler who managed to briefly transcend his class origins and historical epoch, Soviet historiography tended to portray Catherine II as an exemplar of them. Her suppression of popular revolts like the Pugachev Revolt and her intervention against the Frecnh Revolution showed her true colors. In contrast to its positive entry on Peter I, the Great Soviet Encyclopedia noted that

Catherine herself took an active part in government life. A thirst for power and glory was the essential motive for her activity. Her policy was aristocratic in its class direction. In the 1760's, Catherine concealed the aristocratic nature of her policy with liberal phraseology (such behavior was characteristic of "enlightened absolutism"). The object of her lively relations with Voltaire and the French Encyclopedists and her generous monetary gifts to them was to conceal the true nature of her policy. In 1767, Catherine convened a legislative commission and drew up the Instruction for it, extensively borrowing the ideas of the progressive Western thinkers. However, the commission’s work was interrupted in 1768 under the pretext of the war with Turkey. In 1765, the Free Economic Society was established, in the interests of the nobility. A general land survey was begun in 1766 in order to put the landlords’ property rights in order. A series of decrees strengthened the landlord’s authority over the peasants.

Unlike Peter I, Catherine II is not an innovator, but rather a slavish borrower of Western ideas.

One of the things that enabled this dim view of Catherine II to take hold in Soviet historiography and official memory was that she had a class of contemporaries that the Soviet state chose to lionize instead. Although many of them were loyal servitors of the Romanov state, they often come across as historical figures much like Peter I in that they have a pragmatic ethos. Military generals like Suvorov, scientists like Mikhail Lomonosov, or poets like Pushkin became the heroes of the late-eighteenth century and the early-nineteenth century in Soviet commemorative culture, not Catherine II. Soviet writers often emphasized the connections of these men (and they were almost universally men) to various progressive intellectual movements and ideas, showing that they had demonstrated an ability to step outside of the halo of their class and historical epoch to grasp the true socioeconomic bases of the Russian empire. But such recognition could only go so far given the narrow limits of Russia's socioeconomic development, but these figures did recognize the problems and thus their accomplishments were not tarnished by their association with the ancien regime.