So I’ve been doing a lot of thinking about Sokka’s hairdo.
The first thing that seems obvious is that the “Warriors’s Wolf Tail” is what men/warriors of the Southern Water Tribe wear. Hakoda has one, Bato has one I think, and as I recall most of Hakoda’s men also wear one. So that’s easy.
But why the shaved sides? It’s not a sign of his boyhood, because according to Korra flashbacks he wears it that way as an adult, it’s not mourning for his mother because we see in his flashback that he wore it like that before his mother died, it’s not dishonor the way it apparently is in the Fire Nation. But it’s also distinct enough that it doesn’t feel like just an aesthetic choice.
The big thing is that shaving the majority of your hair in a climate like the South Pole is exposing yourself to the cold. The reason why everybody else has “big hair,” especially hair that covers their ears, is that it keeps them warm. So by shaving his head, Sokka is sacrificing warmth and comfort - most people wouldn’t do that just for an aesthetic, not even Sokka.
Thought: the sacrifice of comfort is the point. It’s a sign of his personal strength. Sokka shaves his head because he doesn’t need the warmth or the covering for his ears and neck - he is strong enough to resist the cold without it. It’s a signal to everyone around him that he is tough, he is a survivor, and he can continue on even when the odds are stacked against him.
Conclusion/Headcanon: in the Water Tribe, shaved hair signifies a great trial undergone and overcome.