In my opinion, she seems quite happy, especially with Albert. They seem very in tune, to be expected, but she does have the face that when it goes into repose, or 'rest', defaults to a mode that can be interpreted as 'sad'. I would call it more 'pensive'. The same happens to other royals - Queen Elizabeth comes to mind. When she forgets to stay 'on' her face defaults to looking grumpy.

She has even talked about it - she's aware that it happens. I think the same occurs for Charlene, except the default mode appears 'sad' depending how you 'inform' the image.