Honestly there is a possible debate here about the tone of the message, and I can see a bit of both sides of that so I'm not even going to dig in there.
But I really do feel that much of the criticism here is coming at her regardless of the tone of the message and it's something that I really do find a bit annoying. What I see here is someone who is exceptional, I don't care about debating the value of what she has done (some value fitness more than others) and I don't care about debating how much genetics have helped her or how busy her lifestyle is, regardless of anything else she's a mom with three young kids who is in incredible shape. I personally happen to think that's pretty impressive and laudable, and I would think that most others would agree. And not just laudable, she's pretty clearly put herself far outside of the "normal" expectations of fitness for a new mom.
What I see in this case and other cases is that regardless of the message tone when someone gets media attention for a lifestyle that is far outside of cultural norms they get a heavy dose of criticism from those that feel that exceptionalism is a slight against them, even if they are not in the conversation at all. If her message with the picture was something less challenging like "Moms can get fit too!" or something like that I believe that she'd still get 90% of the backlash that she's getting now. I don't believe that it's mostly the message tone, it's the simple fact that she's exceptional and that creates an "implied judgement" to lots of folks, even if no judgement is involved.
I remember a thread on GAF about one episode of A Life Well Wasted podcast where they covered a programmer who gave up the corporate rat race to move out and live a very simple and cheap self-sufficient life style on very low income. This interview had zero judgement involved, there was no criticism, but there was an "exceptional" lifestyle and a clear move outside of social / cultural norms. And in that thread I remember plenty of the same types of comments that I see directed at Ms. Kang. Since she can't really be attacked in the area that she is exceptional in she has to either be attacked for the possible trade offs that she's made "do her poor kids ever see their mom," "I bet her job is not very challenging," "well sure, if you think that having the perfect body is more important than contributing to society" or by implied judgement "why does she think that everyone needs to have a body like hers," "she doesn't understand why getting in shape is hard for me" and so on and so on. For some people there is just no room for someone to do something awesome without having to either pick at their motives or to project out their own guilt onto them.
Now that fact that her tone was a bit aggressive does muddy the waters a bit, but I'd stand by my comment that if she posted it with a positive message or heck, with no message at all and the picture went viral she'd be getting almost exactly the same flack back.