I haven't heard of any remake of The Christmas Story. Usually, such remakes are announced through official channels or media reports. Keep an eye out for those if you're interested.
The official Undertale content is a carefully crafted story with deep themes and engaging gameplay. It includes elements like the mystery of the Underground, the personalities of the various monsters, and the overarching plot of the human - monster relationship. A 'girl fart story' is completely absent from this official content as it does not align with the tone, style, or themes of the game.
One key factor is its indie nature. As an indie game, it had a fresh and unique concept that wasn't influenced by big - studio trends. It was able to stand out.
The back story of Undertale involves a complex web of events and characters. It includes the history of the underground world and the reasons for the conflicts and struggles that take place.