I've lost about 45 lbs in the past year, and I have noticed a decline in sweating. I can't say for sure how much is because of my weight loss/fitness, because I also just started using Iontophoresis less than 2 years ago. I definitely think it has helped, but not sure just how much.
When you're fitter, you have a lower resting heart rate, which I think helps prevent your body from triggering the part of the body that tells it to sweat. I think looking and feeling better also helps with your self confidence, whcih helps prevent the HH from triggering. Also, because you are eating healthier, you are putting less toxins/junk in your body, which helps.
Overall, it helps to be fit, but I don't think it will dramatically help reduce the sweating, but it will help some for sure. I think the more overweight you are, the more benefit you will see in the reduction of sweating, if you lose weight/eat better/increase cardiovascular conditioning.