"First Lady" is not actually a job. People don't receive a paycheque or any official duties for being First Lady. Most try to use it as an opportunity to try do something good for their country with regards to issues that interest them, but it's not an obligation. Unless you're proposing to change that, I see no reason why we have any more right to see Michelle Obama's thesis than anyone else's.
(I'm not sure if there's a specific incident you are referring to, but where I've gone to school, the university library carried copy of every graduate's thesis, which anyone could look at as long as they didn't take it out of the library.)
