It is brought down that one should avoid having a goy come into his sukkah, because the sukkah has Kedushas HaChil, the holiness of the area of the Azarah in the B”H where goyim were not allowed to enter.
Also that’s why we shouldn’t have a goy put up the s’chach. Because since that is the ikkar part of the sukkah (zeicher the Annanei HaKovod).
It is also called the Tzail DimiHeiminusa. The sukkah is infused with tremendous kedusha.
Therefore if one has a goyish neighbour or friend that wishes to see his decorated sukka, either invite them before sukkos or on simchas torah.