Quantifiable IQ measurements became known and adopted in the USSR decades after Stalin’s death, so we don’t know for sure. What we know is that he was enormously successful in his role as Soviet ruler. Successful people, especially those who can survive and triumph in the universe of Communist politics, are likely to be very intelligent.
Before he became the top dog, no one among his Bolshevik comrades considered him the brightest bulb in the bunch. He was seen as a diligent, but pigheaded and vindictive workhorse, born for back-office paper-shuffling, and not much else.
During the Civil War, the Party sent him to command the Red forces in the Volga region, then in Poland and Caucasus. His achievements were middling. He also caused Lenin’s wrath at his mismanagement of ethnic politics in the south. However, his brand recognition as an unsurpassed back-office workhorse secured to him the new post of General Secretary who was tasked with putting in order membership in the Party in 1922.
This was Stalin’s claim to fame.
He used his enormous work capacity for creating the procedures and network at all levels of power in order to neutralize and eliminate all his competitors, one by one. After that was done, everyone suddenly discovered how brilliant and deep he was. Luminaries from East and West who made a pilgrimage to the Kremlin, and especially our allies during WW2, poured words of amazement at the man. Only solitary voices like Trotsky kept insisting that he was no more than an “extraordinary mediocrity”.
As to Stalin’s theoretical works, none of them are remarkable in any sense. He spoke slowly. His style is repetitive, relatively simple, brimful of trivialities and dumbed-down references to classics. It is debatable how much of this can be attributed to his inadequate mastery of Russian (not his native language), or to a deliberate adaptation to the level of his audience and party comrades. A few poems he wrote, are run-of-the-mill and will hardly ever make it into poetic anthologies of the 20th century.
However, from people who knew Stalin, we can infer that Stalin scored extremely high on work capacity, curiosity, power of observation and ego. In retrospective, we also can also see his outstanding strategic vision. How much this came from his almost photographic memory and extraordinary single-mindedness, or from his IQ, is impossible to say.
Below, one of Stalin’s portraits made during his rule. As you can see, he was not only sharp and very successful, but also had an acute sense of style, an athlete’s physique and was at least 6′3 tall.