Mario is just a plumber who gets really high, and has a pet turtle that he thinks becomes huge and dangerous looking and can talk. The princess is his imaginary girlfriend that in his mind he is the hero plumber to save her over and over, but his girlfriend in his imagination likes being kidnapped and has "fun" (heh heh) by his turtle (bowser) like some sick beastility thing. He gets high off mushrooms and the mushrooms become alive in his mind
yoshi is really just his pet lizard. Which he rides thinking it becomes a giant dragon...his lizard is probably actually dead from being ridden on by a giant. Luigi is just his brother who comes in and checks on him, but otherwise just wants free stuff and mooch off his brother and probably gets high too, but he is the more stable of brothers (which isn't saying much). And the rest of it is just mario's imagination becomes of how high he gets. Like the ghosts probably his fears and regret manisfestating in his mind and become "real" after he consumes shrooms
Mario I'm pretty sure is a game to teach kids not to get high off shrooms or you become a fat plumber who wastes his life away getting really high and living in an imaginary world and being actually all alone, but thinking you aren't by taking shrooms and then suddenly mushrooms can talk, turtles can talk and you become a hero in some weird fantasy world...but while in his mind it is great, in real life, mario must live a sad lonely life.
That is my theory of mario anyway.
My Skyrim, Fallout 4, Starbound and WoW + other game mods at MODDB:
https://www.moddb.com/mods/skyrim-anime-overhaul