If the shunt capacitance is reduced then the string efficiency of an insulator is

The string efficiency is defined as the ratio of voltage across the string to the product of the number of strings and the voltage across the unit adjacent string.

$(String\;efficiency = \frac{{Voltage\;across\;the\;whole\;string}}{{n \times voltage\;across\;the\;unit\;adjacent\;to\;the\;conductor}})$

Because of the shunt capacitance effect between insulators and tower, the voltage across each insulator is non-uniformly distributed.

If the shunt capacitance is reduced then the voltage across insulators will get uniformly distributed. Then string efficiency will increase.