@AndyB The Tangram Heightmapper produces "8-Bit" png heightmaps, with only 256 levels of brightness. It's also has sharp stepping artifacts when grayscale levels shifts.
For terrain rendering uses, the height needs to be in "16-Bits per pixel", float format (or better, 32 Bit EXR). This makes more grayscale levels available, so that stairstepping artifacts do not appear.
For real world data like tangram, you can use "heal" or "Blur" nodes to smoothen, then use Gaea processes to add back natural detail. If that doesn't help, combine with a "constant" at height 0%, blend at zero. The output of combine should be the same at 32Bits, free from stairstepping.