u/Technical-Doubt2076 has thoroughly covered the fashion challenge in concealing and transporting 80 lbs of gunpowder + roofing nails. We can dispense with small quibbles about whether a poor person could afford that much gunpowder, and whether the broad rose-head nails of 1650, used for things like board-and-batten doors more than roofs, could be called roofing nails. The bigger question is the actual effect of all this sewn-in, strapped-and-stayed black powder and hand-forged shrapnel. In Good Omens, touching off the witch at the stake touches off a big explosion in her nether garments. However, black powder is now termed a low explosive because it has low brisance, or shattering power. To go boom, it needs to be contained, whether it's within the heavy paper wrapping of a firecracker or within the heavy oak casks Guy Fawkes placed under Parliament. Assuming there's a lot more nails in that 80lbs ( the volume 30 lbs of loose powder is going to be about 3 gallons) stitched into skirts it's all going to go up in more of a fwoosh. That fwoosh will mean a ball of expanding very hot gas that would accelerate combustion of the witch and quite likely set on fire other things, like spectators. But it would not send nails into the crowd at a great velocity in the manner of a modern-day suicide bomber loaded with high explosives. Whether the witch could waddle to the stake with a 3 gallon keg beneath her skirts I leave to someone else to ponder.