Mariah Carey has a great voice, but that was in her early days. Freddie Mercury is in a class of his own. I can't really remember any of her songs but "Bohemian Rhapsody" is stuck in my brain right now. I guess Mariah should probably bite the dust, huh? She looks a little lonely.
The only one who was on Freddie's level when it comes to live singing was Whitney Houston in her prime! There are singers as Barbra Streisand and Aretha Franklin who come close. Mariah is good in the studio but her live performances has never been that great and she often lip sync or growls in live.
Both Mariah and Freddie had incredible voices. They are potentially the best female and male singers of their time, But when it comes to vocalists of their ability it becomes very subjective. Keep in mind they sing different genres as well.
Freddie's erratic, Rapid vibrato and distortion sounded amazing and can't be compared to Mariah's more silky and smooth tone. Along with Whitney and other great singers who had excellent live technique, It is really just down to preference. It is near impossible to say who was more technically proficient either because any "failings" in vocal technique can be chalked up to stylistic choices.