Lana and Mallow admittedly feel like characters that were better designed to be supporting roles than fellow protagonists. Even worse it felt like their personalities didn't hit off and actually more diluted as time went on.
Lana started off as the introverted 'weird girl' of the group that was understatedly talented, though slowly just sort of became a generic 'exaggeratively good at everything' running gag. Again something that is humorous as a side role but doesn't have a lot of potential as a main character.
Mallow meanwhile started as the genki girl and near enough the leader of the group before Ash got comfortable, and then eventually became just the down to earth 'normal' one, a straight man could have worked, but truthfully wasn't as needed when the other cast members evened out each other already. Again it felt like they tried to rewrite Mallow to be more depthful in the last season but were still kinda lost with how to click her.
By contrast Kiawe and Sophocles started off blander minor characters and then clicked an awesome dynamic with Ash as time went on, while Lillie had a likeable enough character throughout the show but slowly dwindled in having a proper role, just sort of being the cute dorky baby sister for most of it.
With that said however, I can't really begrudge the last season of SM deciding to TRY and give everyone one final attempt at development and not just giving up and throwing them into the sidelines permanently like some of the previous failed companions. I thought Mallow was one of their worst efforts for the first two seasons, but Ultra Legends at least made her somewhat likeable to me, if still not exactly fully realised. I appreciated they were TRYING to fix this issue, compared to earlier where it felt like they barely put any effort into her at all. This is nearly always a deal clincher with me, if I can tell the writers were TRYING, something that isn't always clear with a formulaic show.