Geeky point here, but traditionally Hallowe'en featured men dressed up as women and outlandish creatures of the underworld.
My DS happily dressed as Robin (as in Batman and Robin) for three years running, aged 4, 5 and 6. I made his costume for him, as it was absolutely the only character he wanted to dress as, and it was rally small by the time he was 6 or he might have worn it for a few more years. He graduated to Batman when he was 7 (shop-bought costume). He now wants a career in aviation.
I agree that the costume on the blog was inappropriate for any child and that the Hallowe'en experience was unnecessarily fraught for the child, whereas a little common sense might have made it a happier event. And there's a definite feeling of this woman being contra mundum, which is reinforced by the posting of the child's photo including face.
On the question of bullying that she brings up though -- yes, she's right to assert that everyone has a right to be different, but I think sadly unrealistic to expect that that will happen. I do agree with the implication that when children bully it's the fault of their parents for passing on their prejudices and implying to their offspring that they have a right to pass judgement on others and their choices.