Ponchos for backpacking create airflow to help avoid sweat condensation. The airspace created by the poncho draped over the backpack, arm holes, neck holes, and the open bottom of the poncho help to achieve that.
So the first thing to look at is how much ventilation is one allowing for the poncho? Closed up neck openings and arm openings can exacerbate the problem.
Frogg Toggs makes a poncho which I have used extensively in the rain (my wife used one as well). Compared to my Zpacks poncho, it performs remarkably well, albeit a few ounces heavier. It is made of a 'breathable' material which does provide some help with water vapor escape. It is inexpensive and can be found online at sites like Amazon.
The other factor is how warm one is dressing/ over dressing, and how much energy expenditure and exertion one is undergoing when wearing rain gear. You know. . . the more energy, the more sweat, the more condensation in the rain gear. So dressing light and slowing down while walking can help reduce the condensation problem.
When push comes to shove, being wet and warm from a soaking sweat is far better than being wet and cold from the rain. I hate either choice, but. . .
