Apparently, people argue a lot over who said this. Maya Angelou said it, but she credited Nathaniel Hawthorne. Other famous poets (such as Lord Byron) wrote similar ideas, but who actually said, “Easy reading is damned hard writing”? If you want to read more about the controversy and who said what, I recommend The Quote Investigator Article.
After reading this article and doing a little more research, I think this may be one of those sayings that was adopted into common speech and cannot truly be attributed to one person. And what I think that most people overlook is that to have so many possible authors, this quote has to be something all those authors agreed with enough to quote it (if not write it).
That many authors agreeing on something? Now, that says something.