You can use anything which represents a geometric region, e.g. Disk[]
, Ellipsoid[]
or Polygon[]
:
pts0 = {#1 - .5, #2 + Sqrt[3.]/2} & @@@ First[KochCurve[4]];
pts1 = DeleteDuplicates[Chop@Flatten[NestList[RotationTransform[-Pi/3], pts0, 5], 1]];
Plot3D[Sin[10 x] Cos[10 y] Exp[-4 (x^2 + y^2)], {x, y} \[Element] Polygon[pts1], PlotRange -> All, ColorFunction -> "Rainbow", Mesh -> None, ImageSize -> Large, PlotPoints -> 50, Boxed -> False, Axes -> False]
giving:

Regards -- Henrik