What's the meaning of "Cultural wallop" in the following sentence
- Seldom does a nonfiction book pack the cultural wallop that Dee Brown’s “Bury My Heart at Wounded Knee" did in 1970.
- After all of these years, Wharton still packs a cultural wallop.
In this context, wallop is a synonym for impact. It means that something (in the case of your first example, a book) affected the culture in some profound way.
It's a rather informal way of saying it.
There's the idiom packs a punch, which means "have a powerful effect". In these cases, the writers have substituted wallop for punch – perhaps to give their words more of a wallop.
Incidentally, TFD recognizes "packs a wallop" as an alternative wording for "packs a punch".