My opinions on her aside, I think a lot of the indignation stems from the fact that many mothers are tired of being told that the perfect body is something they have to aspire to. Lets face it, there's healthy, and then there's a perfectly chiseled six pack - something that genetics do play in to, whether or not she was heavier before. If she'd posted a picture of herself pulling the three kids in a wagon in running shoes with a bottle of water strapped to her side and said "what's your excuse?" It would have been a totally different ad.
That ad would have promoted staying active and healthy living as a mother. This ad promotes hours at the gym to meet an aesthetic ideal.