Notice that the comeback is that Ann Romney chose to stay home and raise her 5 boys. Well, plenty of women with 5 kids (or even 1 kid) don't have that choice! They need to raise a family AND work a 40-hour-a-week job.
Yes, being a mom is a full time job. Ann Romney had ONE full time job. The vast majority of middle- and lower-income moms have TWO full time jobs, because they don't have the option of just staying home to raise their kids, because they aren't multi-millionaires like the Romneys.
BTW - I'd be interested to know what kind of domestic help Ann Romney had raising those kids, from nannies to housekeepers, help that the average mom can only dream of.