Obviously underrated. This girl is the definition of artist and talent. Why? She has the star factor, she can sing Pop, Country, Urban, Pop/Rock, Rock, Pop/Electro, Teen Pop songs.... + she can rap.
Her voice isn't overrated, she probably had the best vocals out of any mainstream female vocalist ever when she was in her prime. Her control over all parts of her already big range was insane.
However her discography is very overrated. Mariah has decades of material yet only a small part of it are usable, only a couple of her full albums are even anything above mediocre in my opinion.