If you’re sufficiently good or accomplished at one thing then you’ll be fine. But most students aren’t. Most students who are good at math are just good at math. But if you win two golds at Math Olympiad while in high school or publish a groundbreaking result then your mediocre grades in history will be ignored.

But a 780 on the math SAT isn’t likely to put you in the company of people whose math is sufficient to ignore poor grades otherwise.