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Hot take, apparently, but
a) Percy's hair in TLOVM isn't that bad. It's simple and effective and the built-in shading really helps give dimension to his head. And, more importantly...
b) consider that in the earlier concept art, he does have cool hair. So the hair he ended up with was a conscious choice by Taliesin and Phil Bourassa (and any other artists involved in the process).
Look. He went from this...
... to this:
It doesn't seem all that easier to animate (and if Percy being a pain in the ass was an issue, wouldn't the simplified coat/etc. already accomplish that?), but the main thing that stands out is how much younger it makes Percy look.
Which, y'know, is important, given S1 focuses on his trauma suffered as a teenager. Which was not all that long ago.
He is young, the second youngest of Vox Machina! And a core part of Percy's character is that, despite thinking he and Vex are the only adults in the room, he is not nearly as mature as he thinks he is. Which is also apparent in his more silly moments in S2, which are also complimented by the more boyish hairstyle.
Percy shouldn't be taken too seriously, which I think is emphasized by how much Taliesin makes a point to underline his flaws and bully the character when he can. With the gunslinging and shock-white hair and cool coat and shadow demon and a castle to put it all in, he's already bordering on too cool; making him look like a stud when he's really just a nerd would make it harder to pull off comedic moments with him. On top of emphasizing the horror of what happened to him at the hands of the Briarwoods and Ripley; he's still so young.
And as an audience we need to be reminded of that when he's pulling off a cool pose or trying to flirt with Vex or getting all raspy with the Orthax voice.
Anyways, art is subjective, but Percy as a character walks the fine line between 'coolass anime protagonist' and 'cringefail nerd' and I think the artists and Taliesin knew what they were doing refusing to let him be too #badass or #hot when he's an early-mid twenties nerd wailing that he's very flammable and sniffling pathetically after giving a girl he coat.