A growth mindset is essentially an attitude that hard work and effort pays off. Not surprisingly, a growth mindset results in better maths performance.
If we view ourselves as restricted, with static intelligence levels, there is little motivation to apply ourselves. Those who work hard, will benefit from that hard work as they see it translate into better grades and a better future.
I have seen enough examples now of students who have been told that they are not capable of high levels in maths, and I have seen them confound those same limited expectations.