To me, it is not. I do not think that the problems faced by Jurgis and his family prove that socialism is superior to capitalism. I do believe that they show us that the capitalist system of that time had very serious flaws. However, these are flaws that seem to me to be endemic to human nature. I do not get from the book the sense that a socialist system would be able to cure the problem of greed (for money and power) that is responsible for the social ills portrayed in the book.
I think that the answer to this question is going to be dependent on what you end up believing about socialism. A die hard socialist is probably going to point to Sinclair's ending with zeal and passion because it proves that Jurgis could only find a home when renouncing capitalism and its perverse interpretation of the American Dream. I think that Sinclair believed in the socialist ending of his novel. Yet, I want to pivot the question a bit. While the socialist claim might not be persuasive, like Marx himself, Sinclair is probably more eloquent on suggesting that the current capitalist system, the one being written about at the turn of the century, is in desperate need of repair. His persuasion might lie in his critique of capitalism more than his embrace of socialism. Sinclair points out that someone like Jurgis in the America of "big business" really has little shot of finding happiness and even less of being a success. The work conditions that Jurgis has to endure, the life conditions he must persevere through, and the debasement of Ona, the woman he loves, are all examples of how unchecked and unregulated capitalism can be destructive to both individual and society. It is here where Sinclair is highly persuasive and here where his greatest impact is felt. The solution might be a bit on the romaticized side. Yet, given what he depicted for so much of the novel, it makes sense that he would capitulate to something in which he can believe. However, regardless of what one feels of the ending, the stinging rebuke and critique of capitalism cannot be ignored and it is here where Sinclair makes his greatest impact.