Based on my knowledge, I don't think you can change the tolerance distance of your mouse location. But I think there are two ways may work around to solve your problem. 1. Set the map zoom level more closed to the ployline. 2. Increase the weight of the polyline, of course it may lose the accuracy of the measurement.
I do also this this link may help you better to solve your problem, basically it provide you a way to highlight the line when mouse closed to the ployline.
