The anime seems to take turns trying more plot driven or character driven premises. I think BW was a point they noticed the formula was taking over and tried to give the protagonists more vibrancy (if sometimes in the wrong ways). SM is going an even more extreme direction with that after XY similarly was went back to being more plot and formula reliant.It doesn't really help that Serena interacted with Ash and Clemont who had pretty bland personalities. Maybe if she returns she would have more interesting interactions with the SM group similar to when Dawn returned in BW and had interesting interactions with Iris and Cilan. Dawn definitely was more livelier when she returned in BW compared to how her interactions were with Brock in DP, which were pretty boring and almost non-existent as well.