NWN is exceptionally ugly compared to its contemporaries. The problem isn't the 3D graphics themselves, but the uninspired art direction, the lack of actual three-dimensionality (everything is a flat plane, and different height levels are strictly separated by 90 degree cliffs, making particularly natural environments look horrendous), and the oversized scale of the assets. Just look at the size of the interiors, every tavern looks like a warehouse, the people like halflings compared to the size of the rooms, and you can clearly see the block-based nature of the layout, even moreso than in the Tomb Raider games which are literally made of squares.
NWN was released in 2002, yet it looks worse than your average 3D game from the late 90s. All its contemporaries look better, be it from a technical standpoint or simply through art design. Morrowind is blocky, too, but it has cool art design and is less grid-bound, and has slopes and hills which NWN entirely lacks. Dungeon Siege looked beautiful for the time, with lush landscapes and lots of environmental detail. FPS games like Quake, Quake 2, Unreal, Thief, Half Life, all look better. Now you could say that's because they're first person but the main reason they look better is that everything looks much better proportioned, compared to NWN where everything looks incredibly off-scale. Gothic 2 came out the same year as NWN, too, and it looks gorgeous.
And before you say the oversized tiles and blocks that NWN's environments are made of are designed to be easy to use in a construction kit, I'll have you know that Morrowind uses the exact same system for interiors yet manages to look less off-scale (even though you will notice the repetition of pre-made blocks if you've entered enough interiors). And Morrowind is just as popular a modding platform as NWN, it's in no way harder to use. It also has slopes and hills for the outdoors, whereas NWN only has plateaus and 90 degree angles.
Similarly, Quake and Thief have level editors that allow you to create complex 3D levels very freely, with any shapes you want, angled walls and everything, and the modding scenes of these games show how beautiful you can make a level in these editors. They're also not hard to use at all, some of the best Thief fan missions were made by middle aged housewives with no background in computer tech at all. You don't have to look as shitty as NWN to be easily moddable.
It simply looked shit compared to all of its contemporaries, and it still looks shit now. While other games from the time have been polished up with user-made content to look better than they ever could on old hardware, NWN's inherent limitations prevent it from looking much better than it did. Environment scale will always feel off, there will never be sloped hills, there will never be anything but 90 degree corners, making everything look fake and lame.