Mementos is that one place of where everyone's mind goes when on vacation. But the thing to know is that its a part of the Seven Koopa Hotels and is also everyone's mind world or something like that.
Fake Igor started charging people ¥9001 per second and they all people eventually got tired of paying high prices to live in a train station so then they decided to all leave the hotel at once and then this caused the UnWorld and Mementos fused for like an hour before Joker made everything fine again. Greg Heffley died in this time from being forgotten also. Believe it or not Jack Frost once lived here before the place got torn down.