A user named @Piece_of_Craft shed some light via Twitter, explaining how they had supposedly been contacted by the legal and business affairs division of SIE Europe, as Nintendo had reportedly "objected" to the use of the Super Mario IP in the game.
Now, according to an update on GamesIndustry.biz (thanks, GoNintendo), it's been confirmed Sony is working to take down even more Nintendo-themed creations. Here's its exact statement:
Sony Interactive Entertainment is notifying the affected users that an IP Infringement Notice has been received from Nintendo and that the content will be removed. We cannot comment on volume of content to be removed, this is done on an individual case by case basis.
Seemingly it's the end of the road for all of those Super Mario recreations in Dreams. Even if creators attempt to upload them, from now on projects will be moderated and removed for containing copyrighted material.
It was fun while it lasted, but anyone wanting to play a 3D Mario title with cutting edge graphics is perhaps better off sticking with Super Mario Odyssey on the Switch.