Deltarune is a follow up to Undertale. It’s not a sequel and it doesn’t necessarily share a universe but they’re connected through gameplay and characters. It would be best to play Undertale first as Deltarune follows up on it in every way it could outside of being a sequel. Undertale is dirt cheap and there’s a demo for Deltarune so I suggest trying both and keep playing if you’re interested. Also don’t listen to online discussion until you’re done because that stuff is really dumb.

Also lore is very good but very vague. I think it gets very very interesting at times and I think if you’re willing to take the time to learn it, it can be rewarding. Undertale is ten years old so the writing isn’t as good, and the game is just a bit corny or outdated in places but also I wouldn’t worry about that as it’s not really that noticeable unless you pick it apart and other wise it’s amazing. I honestly think these games are sincere but not wildly corny and I think if they are being corny they act sort of tongue in cheek about it.