Did the "muckrakers" make any significant progress towards reform in the 19th and early 20th centuries?
I guess that depends on what your definition of progress is. "Muckraker" was a derogatory term used by Teddy Roosevelt for activists who dug up and exposed the dirty underside of American life. They were much more likely to refer to themselves as progressives and, often times, socialists.
Publishing books they hoped would expose the abuses of the laissez-faire capitalist system was really the only avenue of protest available for the likes of Upton Sinclair, Jacob Riis, Helen Hunt Jackson and Frank Norris. They wanted to not only inform the public about truly how bad it had gotten in American society, but also to hopefully shock the American public into action to support progressive reforms or even revolution.
As far as encouraging that widespread public outcry, muckrakers had limited success and did help to encourage the reforms of Teddy Roosevelt and William Howard Taft that tamed the trusts and put the first ever regulations on food production and safety. That being said, they did little in terms of achieving any real redistribution of wealth or fundamental change in the capitalist system. Upton Sinclair's The Jungle got Teddy Roosevelt to create the USDA and regulate the meatpacking industry, but Roosevelt thought Sinclair's socialist ideas were ridiculous.