This is a pretty straightforward question.
Before Thor: Ragnarok, Thor had yellow hair.
After having it cut in Thor: Ragnarok, his hair is suddenly brown.
Is it supposed to imply that the arena employees dyed it on top of cutting it?
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Symbolically, it represents change in the character. In the first Thor, he was the bright blonde, naive, and still thinking he was pure. His transformation shows him growing up, becoming more complex. While originally he represented lightness and Loki darkness, they've added shades of complexity.
You can see this in the "Get Help" scene. It's not about two warring gods reluctantly put together. It's about two brothers (insert Rick and Morty) who acknowledge that they have grown apart and traveled different paths. Recognizing that however much they genuinely care for each other, the bond they had is now forever shattered.
This is the same movie where we also see Thor berift of his hammer. And so when you strip the godhood away, you are left with the fundamental man. The point here is that Thor has gradually shifted from his previous haughtiness and godliness to something more like the man he truly is. The innocence has been stripped away leaving a man who is flawed and imperfect, and in doing so becomes more relatable as a person than the previous "incarnation."
At least that's how I interpret it. But from the comments, it was clearly a deliberate choice- take away the armor, the hammer, the blonde hair- and he remains fundamentally Thor. The Thor at the core of who we knew.
Edit: just want to make sure we are clear here that I am not saying that this whole idea of white/blonde=good and dark/black= evil is sane or right. I'm saying that it's a trope that has been used for a very, very long time.