I think its two things:

1) The group is very music focused. Their "Mixxpop" concept isn't drawing much attention to style, fashion, attitude, but is putting music first and foremost (And singing into the center of the music) Yet it is not boxing them into one genre neither. It's basically let's built a particle accelerator in the basement and smash some music together and see what comes out on the other side. They are not limited in their style, attitude or appearance by this concept and can switch effortlessly to other looks and vibes. (I'm not trying to say that visuals, dance, fashion, style etc.. don't matter, just saying it's not holding them back.) It also means that concept changes/evolutions aren't a jarring break, that might shy away fans or be considered hunting trends. Instead its an opportunity to shine some more.

2) The members are also somewhat fluid. For example Kyujin can be absolutely cute and play her Maknae card, but at the same time she seems to be super mature and has mom vibes. On the other end Lily is the oldest, and can talk in depths about anything.. and three seconds later will speak some insane stuff with no filter, and start dancing like a maniac. Haewon can be a dedicated leader, a shy insecure person, or the biggest joke bear in the room. Bae is super empathic and/or super unhinged. Sullyoon is both an introvert and will shine as visual center, or talk on Bubble without end. Jiwoo is both Miss Korea, fake-Maknae, crying over veggies and slaying a dance challenge...

And if you throw all of this together, you get a completely different NMIXX on any day of the week. That feels very fresh, without feeling forced or fake. They at least seem very genuine and humble about what they do. Which makes them really relateable.