It was neither, according to the standards proffered by Ms. Manners, and I'll take her view on matters like these more seriously than yours, based in what I've read of the writings of the two of you.
As stated above, I've yet to encounter a parent in this area who thought that these letters were anything but bullying.
As the old saying goes, "Don't mess with mama bear's cubs"
If a large chunk of the population thinks they wouldn't like it if their kid got one of those letters, whats the percent chance one of those people will be rich or crazy enough to react in one of the extreme ways I outlined.
I think this is one of those areas where the woman may have a RIGHT to hand out those letters, but that doesn't make it right to do so, nor does it make it a good idea to do so.
If her goal is to induce positive change in obese children, championing fitness and diet improvements in school lunches, after school activities for at-risk childern etc would seem to be more positive and more likely to be successful.
Versus doing a bait and switch at the door by advertising she's putting out for Halloween, but then Grinching on some of the customers.