Well I was the one who came up with English translation name for 冥 type. I chose Nether because it a sort of implied connotation. Like the 'Nether' World, which is where much of the Nether type puppets live. This can even work for puppets that aren't necessarily dead, but are dead-like; such as Youmu, the poltergeists, tsukumogami, etc. Nether World is also synonymous with Hell, from which is where the Yama rules. Komachi and other shinigami are subjection to the yama, and well, they bring death. There's also an old hell, which is where Rin lives and she controls the evil spirits of the dead. Since Nether means 'below ground', it also can imply things that can be found 'underground', such as dead, or decomposing matter, like Yoshika. Sekibanki is a rokurokubi/nukekubi. And you know what pokemon is also a nukekubi? Misdreavus. So there's a basis for Sekibanki being a Nether type. There's also the part where everyone liked the name 'Nether' type.
Honestly, if I were to give Nether another name, I'd call Necro, Curse, Dusk, or Murk type. From which, when we were first translating this we were calling it Dusk-type but not many people liked it. So it got changed to Nether.