Deltarune takes place in an alternate timeline, so it doesn’t take place before or after undertale. The monsters were never driven underground. Asriel lived and grew up and went to college. Undyne has both eyes and has never met Alphys, who is more into western comics and cartoons instead of anime and manga (tho she started reading Mew Mew Kissy Cutie recently). Bratty and Catty aren’t BFFs, they hate each other. Mettaton is still a ghost. Toriel and Asgore got divorced for probably normal reasons. Gerson Boom and the Shyren are dead. Sans fucked your mom.
people seem to forget a lot that the dm is ALSO one of the players whenever you sit down for a session, and that their comfort, enjoyment, and limitations matter just as much as everyone else’s. i think sometimes people get really used to treating the dm like they’re a game engine that can and will have to produce entertaining results for anything you do as a player without realizing that while improvisation is a valued and important skill for a dm, its also probably a sign of a bad player to take too much pride in burning your dm’s campaign to the ground. imagine if another player got smug about constantly pushing your pc off of cliffs. you wouldn’t be having a very good time if that happened.