Sweet potatoes have a high moisture content. After cooking they are around 75% moisture, so the flour absorbs the moisture, and the dough gets sticky. When this happens as you make the gnocchi dough and shape the dough, you can sprinkle a tablespoon or so of flour on the dough/gnocchi to reduce the stickiness, but take care not to load on extra flour which will yield an overly dense gnocchi.