Her introduction was awful, linking her to a past thing with Ash is not only overly cliche, but actually takes away from Ash's own character development over the series, and it's just... disappointing. And even a little creepy the idea of her recognizing him (and remembering him that much when he obviously had no idea who she was) and then going around carrying this thing and just the whole thing involving them is kind of unsettling to me?
Also, some of her lines just come off as very insulting to past female characters as well to me. I can understand her finally confessing, but I'd be very surprised (and almost find it uncharacteristic) if Ash felt it mutually. His interactions with her felt no different than interactions with other characters (with the small exception that Serena has a crush on him) and in general, I don't feel they've gotten enough development to actually push that envelope and make it not feel cheap or lazy (though, again, with how Serena has been written, more laziness wouldn't be that surprising). Misty's relationship with Ash (and the idea of them having feelings--while Misty's was very likely/practically confirmed, Ash did show possibilities of feeling the same way--though, it'd still have been a long time I think for him to recognize exactly what those feelings were) was definitely much more developed in that kind of wave, but to be honest, good overarching character development outside of occasional episodes ended when they started Hoenn IMO. :/