My overall problem with Mallow is that she slowly became 'the normal one' of the group quickly in and didn't have as much of an agency. Hell even in terms of comedy, Mallow tended to get the least gags and shirk from the slapstick antics more often than everyone else. I feel like the last season shown some progress in trying to fix this about her, she definitely got more to do in that one, even in terms of little character moments, but she still often felt like she struggled to get involved in a plot compared to Kiawe, Sophocles or even Lillie. You could often pair them with Ash and they'd still have a lot to do, even as a supporting character, while Mallow was too often reduced to the 'prop' character many other series had at least one of.
I feel like the problem was that they too often gave them the allusion of being strong by pitting them against easy opponents. Mallow and Lana too often got their episodes interrupted Team Rocket or some other jobber villain for example, who they always beat effortlessly without taking a single hit. They were at least honest that beating up those wimps meant squat to Mallow and Lillie's experience in the end, but all the same, why have those hijack their plots over some actual development? I'd rather they shown Tsareena either fighting a real opponent or doing something relevent to Mallow's goals, or at least something with more agency than beating up the Meowth line very very easily over and over, or getting a 'power of really really wanting it' evolution after doing nothing to earn it. Only two moments she got were bonding with Shaymin and mastering the Z Move, both instances stemmed from advancing Mallow's character a little.
I do definitely feel they wanted to convey Lana as a battler to some level, but that only makes it more incredulous she never got an official battle until the league. Hell both her and Lillie expressed interest in the Battle Royal Dome but never took part.
Maybe I'm just sour because I was hoping TR's boosts in SM would make them a more dynamic opponent like in BW (albeit with their personalities kept this time). After all Alola already had a stand in for a jobber villain team. But no, TR just seldom used their Pokemon and Z moves, and remained a formulaic padding villain that didn't even offer a challenge to the trainers with zero battle experience.
In terms of the league itself however, I don't think that was breaking the series' code too much, the whole recurrent plot was the whole group taking part in something whether it was their forte or not. The theme throughout the league was that taking part and trying, not winning, was more honourable. Also it was an extra payback for Ash, who lost nearly every sport beforehand while one of the others aced it. Here it was Ash's turn to win while the others struggled.