Typically, Mary is sharing the Power of Shazam either two or three ways, which reduces all of her powers accordingly. If she's the sole user of the Power of Shazam, she's just as powerful as Captain Marvel is when he's the sole user of the spell.
A "full powered" Mary should beat the hell out of Carol. At 1/2 power, she'd still have the edge, but not nearly by a massive margin. If she's sharing the power with Billy and Freddy, Carol could likely win a majority.
Desaad/Black Mary Marvel are different stories, though.