so is there an explanation why she was made a sex symbol despite being frumpy and deliberately not male-gaze coded.

I think patient zero for this phenomenon was the first Live Action movie in 2002 with Sarah Michelle Gellar. She now had normal proportions instead of short legs and long back dumpy frame. Even had a scene where she went sexy? The dominoes started to fall after that.

Mystery solved. Thank you for coming to my TED talk.