There is no definitive evidence to suggest that Albert Einstein explicitly stated that he was a better mathematician than Sir Isaac Newton. Both Einstein and Newton made immense contributions to the fields of physics and mathematics, albeit in different eras and with different areas of focus.
Einstein is best known for his work on the theory of relativity, which revolutionized our understanding of space, time, and gravity. His mathematical contributions were primarily in the field of differential and integral calculus, as well as tensor calculus. Newton, on the other hand, is famous for his laws of motion and universal gravitation, which laid the foundation for classical physics. He also made significant contributions to calculus, optics, and the study of planetary motion.
It's worth noting that comparing the mathematical abilities or achievements of these two great scientists is not straightforward. They worked in different time periods and tackled different scientific problems. Einstein himself held Newton in high regard and acknowledged his monumental contributions to science. However, any direct comparison between their mathematical skills or assessments of each other's abilities would be speculative since there is no documented evidence of such a comparison from either Einstein or Newton themselves.