The early 00's spate of Alpha Bitch-style Asian Air Heads and ambiguous brunettes are Take Thats against Cho Chang.While extremely popular and pretty, Cho is obviously neither a Libby nor an airhead (she's in Ravenclaw... but then again so is Luna), she chose that traitor Marietta over Harry and Dumbledore's Army and an angsty, insecure teen (recent love-interest deaths be damned). Apparently a lot of people didn't handle Harry and Cho's breakup very well.
This trope was meant to put the "All Chinese people are smart" stereotype to rest.Self explanatory.
It's like the New Style Dragon LadyTrope creator here. My personal theory is that basically it was an attempt, conscious or not, to somehow "reboot" the Dragon Lady stereotype of Asian women into something both superficially less insulting and still openly disdainful. In other words, instead of being a catty bitch from China (or Korea, or Japan, or whatever) she's a stupid bimbo from China, Japan, whatever...BUT, wait, it's not a racial slur, look how hot she is! See how that works? The initial Trixie-type, "Screw the rules, I'm pretty!" Asian Airhead seems to have once been the standard while the nicer, Innocent Fanservice Girl, "Aw shucks, where'd all this hotness come from" version came later, possibly due to an unintended fetishization of the stereotype. And lets be honest, characterizing your stereotype as "Well, yeah, she's dumb, but my God look at her!" is kind of asking for fetishization.