I disagree that barely anyone in SM got character development. I think SM's problem is that they haven't settled on each character's issues unlike in the previous sagas where we were given early on what kind of problems lie within the main Pokegirl/boy. Here, the characters have gags and moments of vulnerability but never to the point where it's explicitly shown it really affects them like Lillie's inability to touch Pokémon. It's kind of like they're sheltered children who don't have to grow outside of their bubble. Actually, Ash's Pokémon are the most developed.
Kiawe doesn't seem to have anything that hinders him outside of him being overly emotional but that aspect is played up for laughs. He's shown to have a tendency to take traditions seriously, believing highly in earning what you get. In the beginning, Kiawe was upset about Ash being given a Z Ring by Tapu Koko without any sort of challenge. I thought he'd still hold a longer grudge against Ash for this but seems like it's since been brushed under the rug. As far as we're shown, he's just a competent fighter who hold onto old values, having no issues addressed. With Mallow, she's shown to always lend a helping hand and despite her minimal episodes, I think she does have a bit of character development. Her episodes are focused on her dream of making the Aina Cafeteria the #1 restaurant in Alola. Even though they aren't well received (hell, I'm the only one who actually enjoys them simply because it's about Mao chan!), those episodes have slowly progressed her goal. As far as shining light onto her internal issues, she's overworked and feels underappreciated. She's always the one to help someone but never someone helping her out, which is why she stormed off to Oranguru's bar to rant about her problems. That issue was quickly solved (and should've lasted longer) but it shows that despite her hardworking, energetic self, she has limits and in my opinion, doesn't know when to ask for help when she needs it. I also feel like the writers are unsure on what to do with her.
I think Lana so far has been the most developed outside of Lillie. She starts off having a simple dream of wanting to explore the ocean inside of one of Popplio's bubbles. Popplio had an issue of keeping the bubble from popping. They both worked up to each day by day through practice and once Popplio learned to keep the bubble balloon, Lana took the opportunity of going against Totem Wishiwashi and meeting I'da and the Seafolk members. Popplio also got stronger and eventually, they received a Z Ring and being able to use Z Moves. And sooner or later, Lana will get a second Pokémon. I remember her getting a bunch of episodes dedicated to her. Remember, she's an action girl and gets underestimated all the time.
I don't think I have to explain Lillie for obvious reasons. Sophocles just recently shared us his goal and even though he doesn't do much, Togedemaru does come in handy with her Biri Biri Chika Chika ability... and he has Charjabug that he does use!
