There is a huge difference between reading and writing.

How do you know how the librarian responded the other four times? I can easily imagine something like this happening: the first time was great because they identified this wonderful reader. The second time was equally great because the reader was still at it. The third time was, uh good but how do we get other kids involved. The fourth time was, this is getting to be a problem. Maybe the kid will be a good sport and bow out of the competition next year so that other kids have a chance. The fifth time: snap and inappropriate response.

J.K. Rowling is a totally different situation. I only read the first three or four Harry Potter books, so I can't say whether her writing declined later on or if people considered her a hack because of her success. It is a genuine concern since some people regard anything with mass appeal as being low quality, which isn't necessarily the case. I know that adults frequently become concerned when kids are too heavily invested in reading a particular series, at the expense of reading books in general. Teachers in particular prefer diversity and quality over quantity. Unfortunately, that concern for the kids may have spilled over, inappropriately, to reflect poorly upon Rowling.

That said, an author and a reader differ in vital ways. An author creates something that has an impact upon the lives of other people. The richer they become, the more successful they were at it. (We can only hope that the impact they had on people's lives in the process was positive.) But readers just consume. They don't make a contribution to others unless they choose to do something with what they learned while reading. Even then, quantity is no substitute for quality. Even then, quantities of quality reading is no substitute for those who act upon what they read.

A copious reader may be a high achiever. Then again, they may not be. Simply put, I'd rather call a person an achiever based upon what they created rather than how much they consumed.