Upton Sinclair is writing a plea to his audience for understanding of the ordinary Joe and his struggle against the Man. When you want someone to understand (and agree with) your point of view, you generally don't use really complex or technical language to scare him or her off. So it makes sense that the style of The Jungle is straightforward and easy to follow. Upton Sinclair is doing his best to attract the sympathies of a general audience.
However, Upton Sinclair is also tackling huge topics. He uses Chicago's meatpacking plants as a backdrop for a much larger critique of big business, crooked politics, and social inequality. Even though his language is clear, the plot of The Jungle is pretty involved. There are a lot of twists and turns as Sinclair jams in discussions of everything unjust in 1906 America, from poor quality over-the-counter medicine to cheating midwives to burning rivers. While we would give this novel a two in difficulty for language, the Big Ideas Sinclair wants us to swallow come in at around a five. So we'll average them out and assign The Jungle a four for total overall toughness: you'll have to spend some time meditating on Sinclair's politics, which are a lot harder than his words.