The way mhw shows off Nergigante doesn't really make him seem relevant to the story
You don't encounter him the entire time after the 1st Zorah siege, going through at least two other entire biomes with NO mention of him at all, then all of a sudden he returns in the 2nd zorah siege
You go to the Elder's Recess to find traces of him to learn even something about him, but don't see him until the actual assignment the game gives you?
Now, Iceborne made him slightly better by having a cutscene where his subspecies(?) takes down the Earth Elder Dragon, Shara Ishvalda, but it makes Ruiner look like he's an actual threat compared to regular Nergigante
That Shara cutscene and the Bazel cutscene are really the only mentions of Nerg in the dlc though as far I can tell

I don't know, that's my thoughts on it. He's a fantastic monster conceptually and definitely design-wise, but the implementation wasn't good imo
For a flagship Elder Dragon nicknamed both "Extinction Dragon" and "Eater of Elders", he's just shown to pose as much of a threat as a rampaging Rathalos, and it doesn't make him intimidating