I respect your right to this opinion but I do wonder where that is written as a rule of etiquette.
I've always believed that a person's physical or emotional needs should be factored in, even in the highest diplomatic situations and duties, even when such an edict can be cited.
George and Laura work very hard to deny themselves their own comfort for the sake of others and their country on a constant basis. I'm sure I have no idea the extent of what they deny themselves in this regard. Their privacy and freedom from public raking are two of them.
I am sure she had her reasons for wearing the glasses. I am just so proud of them for being the first presidential couple to attend a papal funeral, for Bush's strong support of Pope John Paul II in his lifetime, and his commitment to keep working with the Vatican.
I think the "issue" with Laura's sunglasses has more to do with their size than the fact that she was wearing them.
Perhaps a smaller, less glamorous pair would have served her better at the funeral.