The story (both movie and book) shows the darker side of capitalism and Stalin hoped it would make great propaganda to reinforce why capitalism doesn't work to make soviets grateful for communism. But instead, the people that saw the movie were just shocked that even the worst Americans were better off than the the average soviet citizen, which backfired on Stalin's intended goal.
More importantly, the ending of The Grapes of Wrath shows hope in even the darkest moments and a way that an American had the freedom to turn their luck around and give their family a fresh opportunity at life, even when they had seemingly lost everything. This showed power that the poorest Americans had, and soviets couldn't ever hope of having. That freedom was surely impactful.