Mario is a semi-comedic character. You can tell just by the shape of his 'Stache® that he doesn't seem to take anything seriously in most games. Unlike characters like Link Mario generally has a very smooth, undetailed texture, usually meaning that the game is more cartoony than hard and/or creepy, which is why Mario games are easier than Legend of Zelda games. However in Super Smash Bros Brawl he has a more detailed texture, and a more intense look on his face, which makes him look more like an actual plumber instead of a guy in his pajamas eating mushrooms and giving funny names to weird characters. In Legend of Zelda games Link takes saving Hyrule more seriously than Mario saving Peach and the Mushroom Kingdom. My point is, creepy pastas and easter eggs aren't exactly Mario's thing, but the only three sightings of these mysterious "trees" are in the ShiverBurn Galaxy, the Grand Master Galaxy and World 4-4 of Super Mario 3D world. Maybe these originated in a more intense game with a more thought-out plot, such as TLoZ. Not to mention Hyrule is a large area with a lot of secrets. There's bound to be a few easter eggs in there. The alien ghost theory is based on another appearence of the Trees in Super Mario 3D world and another easter egg, a UFO which can apparently be seen by looking through a specific pair of binoculars. The Trees are supposedly seen if you stay in a ghost house long enough, leading some to believe they may be a form of Boo. But there are a lot of theories. Some think that they are Endermen from Minecraft. Some think that they are Floos from Super Smash Bros Brawl. Some think that they are a mysterious enemy, who will haunt your games, causing them to glitch and look scary. Some think that they are some dumb easter egg, but clearly they are not. Nobody makes up a character as creepy as a hell valley sky tree and puts it in a game as fun and bright as Mario Galaxy 2 or Super Mario 3D world. As I said, this seems to be some sort of Legend of Zelda monster.