Which isn’t the characters fault. At the end of the day I may love Lillie but she already had an established personality before the series even started. The writers just expanded on it. She always was a timid smart girl who wants to overcome her fears and progress to eventually become a trainer. The writers added their twists and turns but nobody can say game Lillie and anime Lillie are two completely different characters. It’s just who the writers cares about and who they don’t
Which is a fair point but only punctuates the truth, they're just not clicking anything for Mallow.
I think a series with a large ensemble cast can work so long as they find a developed place for every character to stick out. Look at shows like My Little Pony and Winnie the Pooh, which have large casts that are often limelighted as much as the de facto lead. There's always some characters that 'click' better than others however.
MLP has done an okay job making some characters like Rainbow Dash and Pinkie Pie as prominent as Twilight Sparkle, and even adding some new main characters like Starlight Glimmer into the mix, though the likes of Applejack are infamous for gets the scraps of limelight. Winnie the Pooh also tends to give Piglet, Tigger and Rabbit as many key roles as the main character (to the point the previous CGI series straight up made Tigger the second title character). Good luck finding a limelight episode for Kanga however. There's also characters like Roo, Owl and Gopher who, depending on era and creative team, fluctuate between being part of the main cast to barely appearing at all.
I'm sure many of us got introduced to ensemble cast shows with Thomas the Tank Engine as well. Similarly there was a pseudo lead (Thomas), but two or three characters who got palpable screen time and story arcs (Percy and James), a few others that weren't regularly important but had okay episodes (Gordon, Henry and Toby), and one or two that just struggled for spotlight at all (Edward).
I don't think it was just the games that gave Lillie a head start however, especially since the show took a lot of liberties on her story. The writers managed to give her a cute foliage with Ash for example, and they also seem more willing to punctuate slapstick and tough scrapes for her than the other girls this series. Lillie's more strategist role was not developed in the games either, which helps a lot in some of her supporting roles. As said, it might just be something the writers saw and could cease lots of potential out of.
Kiawe was also limelighted as minimally as Mallow in the games, and yet he is a much stronger supporting character than her, and while his limelight episodes are the most minimal of the cast, they do the job developing him well enough. To compare Mallow's previous episode was pretty formula driven (nearly EVERYONE rightfully predicted exactly what would happen before it even aired), her two boosts had no involvement from her, and her most previous supporting role was in the Sandshrew episode, where she pretty much just regurgitated exposition to what her goal is and provided minor comic relief, like what one of the third wheel companions often did in previous series.