This is an interesting phenomenon to me, as it's always seemed -- at least in recent years -- that the popularity of the First Lady was inversely correlated to that of the President. President George HW Bush had low poll numbers; his wife didn't. Bill Clinton polled well; Hillary didn't. The nation embraced Barbara "the Silver Fox" Bush, but voted her husband out of office. Although Americans loved Ronald Reagan, as for Nancy Reagan . . . not so much. You get the drift.
So it's noteworthy that Michelle Obama's numbers are falling. I think there may be two different explanations that are coming into play here.
(1) The Obamas came into office as very much of a "package deal" -- not in the "get two for the price of one" sense of the Clintons, but very much in a lifestyle sort of way. Barack Obama has been very canny about using his family to promote an image as just a "regular guy," and the press has been quick to peg them as the most glamorous twosome to occupy the Capitol since, say, Jack and Jacqueline Kennedy. Given that they are closely linked in the public's mind, then, it makes sense that as the bloom comes off the President's rose, the First Lady might experience something of the same phenomenon. Those who were worshippers at the temple of Obama are likely viewing its goddess with a measure of the same disillusionment as they do its god.
(2) Michelle Obama has worked hard to showcase herself as a "regular mom" -- but in its eagerness to rhapsodize about her glamour and elegance, the press has covered enough of her goings-on to make it clear to people that she's not living the same kind of lifestyle that they are. From the personal trainer to the expensive sneakers to the fashion focus, it's hard simultaneously to be the most fabulous First Lady in recent history -- as the press would have us believe -- and also just one of the girls. And the expensive tastes and ooh-la-la lifestyle contrast with her husband's policies, most of which apparently disdain and resent the kind of privilege that characterizes many of Michelle Obama's doings. That sort of implicit hypocrisy doesn't go down well, especially during a recession.