Guys hear me out please . The reason their hair is like this (especially after Clay's was straight when younger) is because of Viva! Her tendency to braid people's hair isn't touched on in the movie besides "it means I like you" so here I am with a whiteboard and a fresh expo marker
As someone who has its fair share of anxiety and regular Bad Thoughts, I reach up to my hair all the time to fidget. It's great, I'm native so it's thick and tough, and without even thinking I can twirl or tie it around for HOURS. If I have media playing I can zone out in, I can play with other people's hair for hours as well and I won't feel any ache in my wrists or anything
Vivas hair is more erratic than Clay's, so I figure that after the great Bergen attack, she would ground herself by reaching up and digging her fingers through her hair, stressing it out and fraying it so it no longer stuck up straight like pop troll hair seems inclined to do, and one day after zoning in from perhaps an anxiety attack or calming herself from a bad nightmare, she'd find the handful of hair she had grabbed was braided. Happens to me more often than I could even count, when it's bad my bangs will just end up braided or tied into a weird knot without me consciously doing it. And since braiding hair takes slightly more effort than just pulling or twisting it, she'd start intentionally braiding her hair in sections as a stress reliever and calming method
Of course over time it would become second nature, we can see how fast she braids Poppy and Branchs hair in the movie (Poppy braids hers as well, but Poppy only did a few small sections vs Viva doing their entire head and VERY neatly) so asking people she trusts to braid their hair as both a way to spend time with them and an even better way to help quell her anxiety (I like body doubling so I might just be projecting) and since Clay is absolutely her best friend, whether you think DreamWorks tried to imply romance or not, he would be the most common "victim" of Vivas braiding and so over the course of two decades their hair has just become accustomed to bring braided and so it's wild like this.
This is very stupid thank you for coming to my lecture
