I get you are not pro-female lead being GAGA over MC But Serena was much more than that. I have to say her standing up against her mother for what she believed in was pretty nice to watch and something we can all relate to at times. Sometimes our parents want us to do what is right for us vs what we want to do because it makes us happy. Serena discovering herself and fighting the best she could for it was inspirational. She even went through a meltdown for goodness sakes. Serena is a girly girl and that's just that. But I think her trials and tribulations overshadow her "ditziness".
I don't think it's ditziness that got on his nerves but more trying to convey such feminine traits as more the core of her character, in his eyes at least.
The elements he listed for May and Dawn for example were foibles, character flaws that made them more vibrant as well making more obstacles for them. Serena was shy and under confident, though besides her crush that obviously could only go that far, it didn't really play that much into plot and development any more than the other girls, who similarly had confidence issues at times as well as other defining personality issues. XY did have a bit too much of a 'good for you' vibe going for it, not really letting the flawed side of the protagonists take hold of the story much or have too many long lasting consequences, which I think maybe hindered their ability to improve that much, compared to previous sagas where they tended to give the twerp cast very hard reality checks.