Well, mermaids do have obvious genetic differences (the fish tail and ability to breathe in water) that would make them better swimmers than humans and humanoid

Well, mermaids do have obvious genetic differences (the fish tail and ability to breathe in water) that would make them better swimmers than humans and humanoids. But I'm struggling to come up with what genetic characteristics about D&D dwarves you assume would make them inherently good at building things. There's nothing obvious that I'm aware of.