I have to admit, even I'm not sure what a "hacker mindset" is. As one technical cofounder to another, I think the most important thing is for you to understand the business side. Most importantly, how interpersonal skills matter and how emotions play out in a business setting.
Beyond that, a solid understanding of scientific approaches to understanding is the second most important. Being able to tease apart correlation and causation, and being rigorous about what you accept as real knowledge vs mere opinion or anecdote. The business world, and the tech world, has a lot of "opinion" that masquerades as fact. E.g. "well I did X and I'm successful so clearly X must work!"
Beyond that, a solid understanding of scientific approaches to understanding is the second most important. Being able to tease apart correlation and causation, and being rigorous about what you accept as real knowledge vs mere opinion or anecdote. The business world, and the tech world, has a lot of "opinion" that masquerades as fact. E.g. "well I did X and I'm successful so clearly X must work!"