The designs of the main characters are fine in my opinion. I like that they are relatively reserved and that you then have the choice to make more radical customization choices as you see fit. I would also like to mention yet again that I'm glad customization is here to stay and that GF appears to have completely abandoned the idea of it being a removable/occasionally occurring aspect after the backlash for ORAS.
Prof. Magnolia is a fine design and I'm glad that we are getting an old professor again, not to say I haven't liked the younger ones we have gotten lately. In particular I like her cane. However I got to say that I would have preferred her field of expertise to have been a bit broader than just the generation's gimmick. Like how Kukui specializes in moves, not just Z-Moves.
Sonia being about as old as the champion and not as old as your rival and you is a new direction if they are planning on making her a travelling companion, so I'm wondering how the dynamic between her and the player will be given this. I like that her design is pretty grounded in reality.
I honestly love Leon's design. I usually don't think sport outfits look that great but the contrast to the very regal-looking cape sells it for me. I like the color choices here too, especially the colors for the cape, and the hair-skin-eye dynamic. Not fully sure if I think the cape being filled with sponsors on the back is just tacky or hilarious yet though. Also I'm less than pleased that Charizard appears to be his ace. I'm kind of guessing it will have some new gimmick like Diantha's Gardevoir but I still think they could have picked a less exposed or a brand new Pokémon for him.
Hop is the least interesting character so far. We have had several rivals in the past with big shoes to fill. His hairstyle is also silly.
The lip movements are an odd addition when the game does't appear to have much if any voice acting. I fear it will take you out of the experience instead of drawing you in to see lips moving with no sound coming out. The more expressive faces might also appear odd appearing side to side with the main characters, which while not as bas as in previous games still appear rather stiff expression-wise. All in all a weird choice both from a design perspective and resource perspective.
Milo's design is a bit jarring compared to the rest so far. His facial features are very different from the rest of the characters. And aside from the actual design being rather different the artwork is drawn by Take(person who drew the Aether guys for SM), who has a very unique style, making it clash even more.
I like Nessa's design a lot, aside from the life rings on the shoes that I think both look silly and dangerous. In particular I like how her eyes just grab your attention. In-game her model looks fine next to the player character's. But her artwork also appears to have been drawn by Take, if you look at her artwork next to Sonia's artwork the differences in artstyle are clear and they hardly look like they belong to the same game. It will be interesting to see how more of Take's art will blend in with more "traditional" Pokémon artists' art if they draws the rest of the Gym Leaders too. And as I mentioned earlier I'm not terribly fond of sport uniforms, and while I think they have been fine so far I'm worried having all the Gym Leaders (and potentially the E4) dress like that will be too much for me, even if I think having a common fashion theme is a cool idea.