Nomo came up with the story ever since the start of NN and Sierra wrote it down for him. The drop in quality is didn't happen because Sierra isn't here anymore to come up with new story, but rather because of the skill difference between Sierra and Monsinne to make a story immersive.What story? What characters? the characters are the same body type with a different face. The story has been incoherent and inexistent, since Sierra left.
A good way to check this is to look into the patches that had little to no influence from Nomo, that were either solely up to Monsinne like the last two filler updates or the two patches where he kind of got out of hand and "wrote more words than any patch before", AKA the human tavern content and the new "orc training" stuff. The fillers are fillers because they don't have Nomo at hand to lead the story and the other two updates were significantly delayed because Monsinne got into a writing frenzy.
At the very least i think that these 4 recent patches show another significant drop in writing quality and especially in immersion.