For those that don't know who Aya is exactly, she is the popular Seiyuu that hit it BIG with her voice of 2006's The Melancholy of Harhu Suzumiya and jPop. Here's a wiki about her other voice roles and her life as a whole -
Aya Hirano - Wikipedia, the free encyclopedia
Is it possible that Otaku have exaggerated Aya excessively? If not, why do you think that?
I personally dont think that she is overratted, hands down she is my favorite Seiyuu even before her role as Haruhi. She started young - at least I think they don't normally start young - and has just recently hit it big, which is why I think she isn't very overratted, but as the way most Otaku are, she may be one day.