We should bear in mind that THE JUNGLE is part of a genre known as Naturalism. This school of literature was intended on focusing on the stark, ugly side of life and taking into account the conditions and circumstances that shape people into what they are. Focusing on what is disgusting is part of its purpose.
I can't think of a school of literature of relatively recent vintage that has had a greater impact on effecting social change. And since labor is one of its major concerns, the unaccounted for abuses of capitalists is one of its prime targets.
This book has resonated with those who suffered at the hands of the rich and powerful. Needless to say, Naturalism is a left wing phenomenon. Even those of the leisure class were shocked by its graphic descriptions.
Naturalism by definition is selectively photographic. Its depictions of life's aspects are incomplete. It has run the gamut of muckraking and reflected a period in Europe and the Americas when it was sorely needed.
If THE JUNGLE is a polemic for socialism, so be it. To avoid a book because one doesn't like the author's politics or religion is to have a closed mind. I would readily prefer the exagerrated injustices of Upton Sinclair over the crypto-totalitarian, stomp on the poor nonsense of Ayn Rand anyday.