Because her first episode had her very active and hr Bounsweet showed a lot of strength from the get-go, whereas Serena's Fennekin almost never fought for real
Fennekin shown a lot of strength from the get-go by saving Serena from a Vespiquen, just after that she never really elevated to being able to take on a target that, you know, didn't run or go down in one move. Even the one time Meowth resisted her she looked screwed. It took till her VERY last battle to take on a developed two sided match with an active opponent, and even there she was clearly the weaker Pokemon. The sad thing was that was still probably the best track record out of Serena's Pokemon.
I don't know what they're going to do with Mallow and the others (though I at least thought it was weird even Kaiwe didn't intervene the last episode, we KNOW he has battle experience and is pretty no nonsense about swatting annoyances away), but I hope they're not going to spend the entire series just doing nothing or not developing past managing little cheap shots, especially if the show is still going to always force them into the middle of Rocket/Skull face-offs like they did Serena. Unless a Pokemon is forced into her ownership, I can sadly see Lillie ending up the load or a damsel in distress a lot of times.
It would be interesting if not every TR face-off was a battle per se and had the heroes stop them through their own little talents and scenarios though. That would be a cool way to dilute the repetition.