There is also the point to be made that one of the core reasons for the poor initial showing of the Red Army against Finland was that Stalin absolutely underestimated the Finns' ability to defend themselves. Not only did he believe/hope that the Finnish left was very close to rising up in another revolution, he also seems to have understood the Finnish military as no better than third-rate White militia, and that just the Red Army opening fire with artillery and Soviet aircraft bombing Finnish cities will bring the nation to its knees in days.
IOTL, only one side in the Winter War was prepared for an actual war in late November 1939, and it was the Finns. The Soviets were prepared to make not a campaign but merely a show of force which would collapse the Finnish "White militia" while the Finnish Reds would again take over in the Finnish towns with the Red Army supporting them, occupying the nation with impunity.
If Stalin still believes these things ITTL, and decides to micromanage the resources etc. allowed to the Leningrad Military District for the Finnish campaign, any even brilliant commander leading the Soviet attack might find himself and his attacking force too small and inadequately outfitted, hamstrung by Stalin's decisions, that an initial curbstomp will not be possible under the circumstances in which the Winter War would be fought. The terrain, the weather and the Finnish competences will still be the same ITTL, after all.
Of course after the initial reality check, ITTL with a competent leader in charge of the campaign in Finland, the Red Army's warplan and its attacking formations would be amended quicker and it might not need all of three months to beat the Finnish army. But even if the Soviets here eventually break the Finnish defence ( in late January 1939, say) and reach Helsinki, many in the West would still say that the Red Army performed badly in the first weeks of the war, and it might well be that Hitler, for example, would still see the Soviet campaign in Finland as supporting his views about the inferiority of the Russians. And that would be down to Stalin misjudging the Finns, not only the ineptness of the Red Army attack - as it partly was even IOTL.