The Konosuba light novel is not a fanfic. It has its own unique plot, characters, and world-building developed by the author's imagination and creativity.
The Konosuba light novel is good. It has a charming mix of fantasy elements and comedic situations that make it highly enjoyable. The character development is also well done.
In the ending of the Konosuba light novel, Kazuma and his party manage to defeat the Demon King. Along the way, they've had many comical and adventurous experiences. Kazuma remains his somewhat lecherous yet heroic self. And they all kind of find their place in the world they've been adventuring in, with relationships between the characters growing and evolving in the typical Konosuba wacky style.