Mathematics Stack Exchange as an indicator for academic career
The only thing that a high MSE or MO reputation indicates is that a person spends too much time on the internet (and I say that as someone with a reasonably high reputation on MO).  I don't think you should take it seriously as a data point on how successful you are likely to be.

On a related note, it also doesn't play any role in things like hiring decisions.  At all the places I've worked, someone would be ridiculed if they brought up MO or MSE reputation at a hiring meeting.

It's just recreation.