To what extent is MSE reputation an indication of potential in academia?
Very little. You get a lot of reputation for simple answers to simple questions that make it onto the HNQ list, because those are the ones that people can quickly understand and say, "Yeah, that's right." Conversely, you get little reputation for deep, detailed answers to difficult questions, because people (probably rightly) think that reading and digesting them won't be a productive use of their time.
High reputation on a technical Stack Exchange site might indicate that you're a good teacher of that subject, since well-explained, easy-to-understand answers tend to get higher reputation. But, the teaching experience you get as a student and early-career researcher will be a much better indication of that.