What is the origin of Boos? Dead people or they just are?
Was the origin of Boos ever touched on?
Are Boos the common myth of ghosts, where they would be spirits of the living that died (like old toads or something) or are Boos just... Boos and have no relation with being a dead person?
According to the https://en.wikipedia.org/wiki/Boo_(character) :
In an interview with Nintendo Power magazine, Mario franchise creator Shigeru Miyamoto stated that while working on Super Mario Bros. 3, co-designer Takashi Tezuka had the idea of putting his wife in the game. According to Miyamoto, "(Tezuka's) wife is very quiet normally, but one day she exploded, maddened by all the time he spent at work.[...]"
Boos appear as white, spherical, levitating ghosts, similar to the will-o'-the-wisp phenomenon or the Japanese Hitodama
So it seems that they are inspired by the "Will-o-wisp" folklore, rather than spirits of the dead. The Will-o-wisp folklore makes them out to be mischievous beings, often leading travelers to traps in the bogs and swamps, (depending on which tales you read, that is).