^I can definitely understand people's frustration with less consequential elements of presidential life. Nonetheless, I think a big part of the appeal is that it gives people something brighter to look to about what's going on in politics. There's only so much depressing political news people can handle. Discussing relatively minor details makes it easier for regular people to relate to the First Family, I suspect. If anything, the gossip about them seems reminiscent of everything I've heard about the Kennedy Camelot years.
I'm not as sure about the coverage of Michelle's clothes, which I haven't really been following, but stuff about the dog seems like a light spot that people can look to in difficult times. Also, not every American is going to read about political topics that they may or may not understand, but the dog is something that everyone can understand. As such, it's a way for the White House to get positive PR that reaches out to a lot more people than would be possible with actual policy items. Basically, soft news will always sell, even when it's about important people. The Obamas are the most important family in the country (probably in the world) at this point, but, as such, they're also huge celebrities.
That said, I don't think that this type of news will ever surprise me. In DC, stuff like that is what makes up the gossip page. We don't have Broadway or Hollywood; our celebrities are, for the most part, of a political sort, so the 'Reliable Source' (the Post's local gossip column) has always centered around that sort of detail in politicians' lives. We had coverage of Laura Bush's wardrobe, et al as well.