How did Japonism, if at all, affect manga/anime?
Short answer is no. Beyond making the world aware of the early, pre-war concept of manga, there is little to nothing to do with modern post-war manga. The influences are just a footnote in history in terms of European art movements. 

Post-war manga we know as manga today is starkly different from  the prototypical  manga of the type, such a the compiled work of Hokusa Manga.