In France, shortly after a president is elected, his official picture is taken. There is only one per president. It is hung in tens of thousands of official places all over the country (in mayors’ offices, for instance). I’ve picked the faces from those photos and put them together. It goes from Charles de Gaulle in 1958 to our time. As you can see none is smiling (Giscard d’Estaing let us see his front teeth but it cannot be compared with the near hilarity of some of your presidents). But, as I said, each of those pictures is the single official one. It is some sort of institution and that probably explains that serious faces are still expected there.
Maybe the absence of smile in the portraits of the past could also be attributed to technical reasons. It probably would have been painful to maintain a smile for the long time necessary to take a photograph back then. And it would have been far worse in the case of painted portraits.
I’m just mentioning that. I don’t think it fully explains the invasion of smiles any more than dentistry’s progress does.
And I don’t know about you, but I cannot imagine Christ smiling as Obama does. And I don’t think he has ever been represented smiling this way in any valuable work of art. He could have been because no posing time or bad dentition was involved there, but he wasn’t.