It is hard to make the argument that Riis, by himself, changed America's moral values. However, the "muckraker movement," of which Riis was a part, certainly changed American attitudes towards social problems.
During the late 1800s and early 1900s, Riis and other muckrakers did a lot of work to expose social ills to the general public. Riis, for example, used his journalism and photography to show the plight of immigrants in big US cities. By exposing these conditions, Riis and other muckrakers made people more aware of the problems and helped make them care about such things.
The muckraking that these people did helped to change American values enough that the Progressive Era was made possible. People stopped thinking that these social ills were inevitable and instead thought that they were wrong and that the government should do something about them.