If you put the solder mask on the radiating element, the frequency band of the antenna will be lowered due to the dielectric constant of the solder mask. Furthermore a portion of the received (or transmitted) wave will be absorbed by the solder mask due to its loss tangent.
This in general, but if the frequency you are working is less than some GHz, these effects are negligible while you will benefit the protectiong action of the solder mask from the oxydation.
By the way here we call it "solder mask", but more frequently "solder resist"