Stalin's ascent to power had very little to do with building up a mass movement through charisma or public speaking. Dictatorship is a little bit more complicated than "one man who ascends to power." Barbara Gordon's typology for dictatorships identifies four main dictatorship types (Military/Single Party/Personalist/Hybrid) which is useful to understand Stalin's rise. The Soviet dictator did not rise up through a parliamentary system like Hitler, which required considerable public charisma and other personal attributes, but rather through an existing dictatorship under Lenin and the Communist Party. Operating in a single-party environment allowed Stalin certain advantages. The Lenin period witnessed the growing importance of the Communist Party and its executive leadership for decision-making in the nascent Soviet state, and Stalin was one of the chief power blocs within the Soviet executive bureaucracy. As his title General Secretary suggests, he was willing to take on bureaucratic drudgework no one else wanted to do. This was one of the critiques Lenin's testament made of Stalin was that he represented something of a bureaucratization of the Party, as well as being excessively rude. But Lenin's testament also was pretty much snarky about everyone in the executive leadership, so it was buried. Lenin's death led to a power vacuum, and Stalin's abilities as both a bureaucratic manager (his nickname was the "grey blur") and his ability to hold his cards close to his chest with regards to his exact political views meant he was courted by multiple sides in the post-Lenin period. Stalin was able to capitalize upon this support, as well as latent antisemitism within the executive leaderships (his other rivals like Trotsky were Jewish) to become a vital bellweather in debates about the direction of the Soviet state. This entire process allowed Stalin to build up both an executive entourage close to him ("Team Stalin" in Stephen Wheatcroft's memorable phrase) and purge out rivals whom he could not control. Once this process was complete by the late 1920s, the cult of personality began to develop in the USSR, and it grew as did the ambitions of the Soviet state.

It s important to realize that Stalin's relationship with the cult was complex and ideologically freighted. One of Stalin's entourage, his adaptive son Artyom Sergeev, recalled a time when Stalin's biological son Vasily used his name when trying to evade police arrest during a drunken binge, told his angry father "But I’m a Stalin too." To which Stalin angrily replied:

No you’re not. You’re not Stalin and I’m not Stalin. Stalin is Soviet power. Stalin is what he is in the newspapers and the portraits, not you, not even me!

The cut of personality around Stalin was not so much directly tied to the man and his vanity, but various state projects. As good a Marxist, Stalin would reject any real idea his leadership was a personalist dictatorship like Hitler. Thus elements of the Stalin cult usually had a state component to them and tended to evade personal myth-building common to other dictatorships. When Beria produced a fawning book about Stalin's early life culled from surviving contemporaries of his youth in Georgia, Stalin saw very little use for it. While this functionally did not produce different results than other dictatorships, Stalin's whims and capriciousness could be quite destructive, the tone of the dictatorship's cult was rather different than its contemporaries.

Finally, as for Stalin's bad language, he was far from alone in this regard. The charter generation of the Bolshevik leadership almost seemed to pride itself on rough speaking and gruff mannerism. Intellectualism like that of Trotsky or Bukharin was often associated with too much abstract theorizing and not being a man of action. Use of the familiar form or rough language was the sign of men who were not burdened by an effete tsarist past and keeping language gruff represented the alleged ascent of the toiling masses into positions of authority and power within the Soviet state. Stalin's semi-official nickname, Vozhd (the boss), implied a more down to earth and practical leadership than more elevated titles. This emancipatory vision not not only explains some of the crude public language among Stalinist officials, but also why many of them saw no need to correct these habits while in power. While Stalin's use of language may have seemed dumb and crude, as a mechanism for achieving power, it was anything but that.

Sources

Fitzpatrick, Sheila. On Stalin's Team: The Years of Living Dangerously in Soviet Politics. Princeton: Princeton University Press, 2015.

Kotkin, Stephen. Stalin. Volume 1, Paradoxes of Power. New York: Penguin, 2014.

Plamper, Jan. The Stalin Cult: A Study in the Alchemy of Power. Stanford, Calif: Hoover Institution, Stanford University, 2012.

I must reject the premise of the question, Stalin was certainly not dumb and for the majority of the time, the majority of the people did not think he was repulsive.

A lot of what the west think about Stalin came from the people who had to leave the USSR because of Stalins policy. These people tend to make him out to be some sort of braindead idiotic monster, no grasp at theory, terrible speaker and microwaved cats as a kid.

Most of this is simply untrue. Stalin want from a poor family and worked his way up, he could have managed to go all the way to the highest education possible in the Caucasus and quite possibly beyond (such as the University in Moscow). He however made the choice to become a priest. This was also a demanding education.

In addition to his formal education he was a autodidact, and he would read quite a lot. Once in the seminar he gets introduced to all sorts of illegal literature. He gets into conflict with the monks because of all this illegal reading and being part of a book club.

From there he goes on in the revolutionary underground. He was never the highest intellectual of the party, but he did publish some relevant works that other people took note off. Underestimating stalin has a long tradition, but he knew his stuff and could and did go up against people like Trotsky at official meetings. There are cases were a large number of people that are not in Stalins group point out how excellent his speech was.

That said, public speaking is definitely not his greatest gift. We know however that in private setting he was extremely charismatic. We have to look no further then to Western leaders talking to him, Churchill and Truman both said that of him. British WW2 Generals Alan Brooke (he is the British counterpart to Marshall and one of the outstanding generals of WW2) who accompanied Churchill to Moscow and he was also extremely impressed with Stalins grasp of military mattes. He recounts a story were Stalin tells everybody to shut up and took over, discussion complicated military subject with a lot of insight. This is specially relevant because Alan Brooke is quite well know for calling people idiots in his diary (Marshall, Eisenhower, Churchill, Pound).

His greatest gift however was being a politician in bureaucratic system. He is quite simply amazing at it. Honestly Stalin is the gold standard of dictators, nobody has ever or has since managed to take and keep so much power. Of course a lot of extraordinary things need to happen for a kid from the borderlands to reach far higher then any Car ever did, but at the same time you can not make that clime without some skills.

He was amazing at controlling his coalition and to divide the opposition. He was amazing at organising people in a way that would make it really hard for them to work against him. People who disliked on another were set to watch each other. He would work around leaders and essentially take away de fact control of organisations.

Source:

• Stalin: Volume I: Paradoxes of Power, 1878-1928

There is a lot of detail about how he eliminated all the different people if you are into that kind of thing.