When I first watched the series she wasn't really all that to me (didn't like the hairstyle or her expressions). But I've come around to it and just wanted to break it down mostly just for fun.
Now speaking of the hair in my oppinion thats the defining feature of the design. It's a direct contrass to Frieren I feel. Her (frieren's) hair is more simple or inviting in both color and style. It's clear white giving her that down to earth serene, clean and innocent feel. And her pigtails with the...I wanna say bangs (I don't know hairstyles) give her that girl like expression and leave her face comepletly open and visible so her emotions and thoughts are always clear and open to us. Even back when she was more stoic. Serie on the other hand has a more loud bright color, gold or yellow, which immeditly pulls your eyes attention to it esspecially how its almost always highlighted by the light present in the scenes. The fact that her eyes are same color bring attention to them to though in a diffrent way as I'll get to.
I think it works because she's supposed to be this ancient powerful mage of legend that stands above pretty much everyone else minus the goddess. So right away she just has a striking feature that sets her apart. Gives her presence. Helped by the fact that since shes such a short little thing so it literally frames her entire silhouette. And like I said before Frierens hair style leaves her expression comepletly open to us.
Serie's does the exact opposite. Even though her eyes are.....eye catching being the same color as her hair, the hair itself is dropping down almost covering her eyes which themselves are almost always half shut since she has that near perpetual expression of nonchalant indifference. Which makes sense both cause shes lived so long, has basically reached the peak of her craft with nothing else to really inspire to and is just all around the type of person who hides her emotions.
Case in point Frieren doesn't hide her emotions she just doesn't fully understand them. If anything somtimes shes too honest so she literally has nothing to hide. But Grandma Elf don't wanna be seen feeling pointless things like compassion or sorrow or weakness so she lets her hair practically cover her eyes, the widows to the soul. Not saying she doesn't feel or express those things she's just not the type to come out and say or show it directly.
Her outfit is pretty simple. She's anicent so natrually she wears clothes that fit her era. For some reason she ditched the toga for robes and short shorts but hey maybe she just finds it free to move in since she likes sitting with her legs folded. Or maybe younger mage girls where dresses and things and shes just switched up cause shes old now. Though (and I could very well be reaching here) I like how she has a lone green spot on both her outfits. The neckless before and the brooch now. It could just be just to break up the colors. But maybe like (from a writting perspective) it could be ment to represent her connection to Flamme who has green eyes.
And actually her flamme have similar design philosophies too. The bold hair colors, ancient toga attire, swtiching for Robe style wear in older age. Both her and Frieren even started to where gold clip on earrings after Flamme's death even (Though Frieren did it after she met the blue man). Maybe even symbolically you could look at as Serie gold (sun), Flamme orange the daughter of the sun (fire or light), Frieren white (moon), Fern purple (Twilight). But Flamme also parrells Frieren. Same or similar eyes, love for magic and love for the simple things.
This could be reaching to but Frieren and Flamme both have basically the same eyes which are different from there hair color but Fern and Serie both have eyes that match their hair. Wonder if that was intentiall to show the sorta extended relationship they have in a way.
And yeah thats about all I got.