Um. She's one of the world's best selling artists. The second highest selling woman artist of all time, behind Madonna, and just above Celine Dion, and the world's best selling artist of the 90s. She's sold 200 million records while Lady Gaga has sold 15. Mariah Carey has been popular over 20 years now. She's won 17 World Music Awards, and I'm fairly certain Lady Gaga hasn't won any, although she does match her in terms of Grammy Awards.
Mariah Carey isn't a "omg shock you look I'm in the spotlight look at me look at me" kind of artist. If that's the kind of artists you listen to, then you haven't heard much of her music because she just makes music. Her demographic is above the people obsessed with Lady Gaga, to an older audience who isn't looking for shock value in their music. You don't hear about her because she's not out there shaving her head, wearing ridiculous outfits for attention, etc. Even her sequel Christmas album, the kind of thing very little people care about, was in the top 5 on the charts when it was released.
Mariah Carey has more of a running at the Queen of Pop than Katy Perry and Pink combined. They're both just blips on the radar compared to her.