ESo to say, Serena is disliked for the fact that she barely did anything in the first half of XY while everyone is just disappointed in Lana and Mallow is simply masking the same problem but with different words because there were tons of people who wanted to see Serena shine but in a more interesting way. Just like there are tons of people who think that Mallow's character episodes are boring and filler just because she didn't catch another Pokemon or learn a new move when her goal IS being progressed. That can be compared to Serena trying out something new in the beginning until she had her breakout episode (episode 60) so at least she had that going for her. Trust me, there are some users who don't like Mallow for being a boring genki girl, forgetting that she had a good supporting role throughout the saga so far. Or what about Lana taking the Totem School Form Wishiwashi from Ash and missing the opportunity to catch Dewiper? We're already dealing with Lillie stealing Alolan Sandscrew from Ash!
I think there is also another problem for all these characters in that while they do have limelight episodes where they do things, they actually don't get to do a lot as supporting characters. One could argue they have a Team Mom role, but it's seen in limited doses and there's not a lot of times they get to demonstrate that through their Pokemon or abilities, besides cooking.
Even concerning her searching for a goal, Serena only had so many episodes revolved around it or developing her team, otherwise in cases where a supporting role was needed in some intuitive personality-strengthening way (eg. not something basic like stopping TR with one or two attacks), Clemont tended to take that role due to his smarts, or in some cases even Bonnie, since the fact she only had Dedenne and her own mischief sometimes made for humorous or inventive situations. Ironically Serena didn't even take the damsel in distress or 'Worf Effect' role very much since they didn't seem to like having her Pokemon lose or take hits too often compared to Clemont's (poor Bunnelby). Serena's one niche, performing, didn't really take some sort of character agency outside the actual showcases (her team had hints of a choreography based battle style, but never developed enough to become commonly useful, again they were rarely put in even handed enough battles to require them), which combined with how formulaic and action driven XY was, meant Serena didn't have quite as unique a function within the team as regularly. Even in more laid back scenes, the team got on too gently (besides maybe Bonnie who just got away with being a brat
) so Serena couldn't even take a non-physical role as the team heart/mediator.
Disregarding her own agenda episodes, she was just the 'girl' of the team.
I could take that with a grain of salt, since it did at least mean that the series was SORT OF evened in spotlight with the companions, with Clemont getting a decent run in the first half before Serena's goal got established in the second.
The same sort of problem lingers on in SM. Some of the characters have some sort of distinction in role, even if it doesn't really come into play as often as it really could (eg. Lillie and Sophocles occasionally being the brains of the function), but out of them Lana and Mallow tend to suffer the most, and usually only get the scraps of involvement or reduced to cheerleaders, unless it's their actual limelight episode (and even there Mallow seems to keep getting eclipsed by another character like TR or her brother). Also keep in mind so far that Mallow has only had FOUR limelight episodes stretched through of 77 so far. Compare to Kiawe, whose only had THREE, but is regularly kept active due to his Alola knowledge and training with Ash, as well as his three Pokemon being regularly helpful in many group fiascos (that he is supposedly sharing the league arc with Ash would also mean his development throughout the series may also culminate in some satisfying way).
It is a shame, cases like the Pollosand or Celesteela episodes look like they're starting to get the hang of it but like Serena's cases, they're far too sporadic to keep them active.