Yes, he looked shabby, but that's not exactly news. It's easy to make it out like 'it's only a coat', and to a certain extent that's true. But it's about respect and 'fitting in'- if the Queen and the rest of the Royal Family can get the dress right, then is it too much to expect the Leader Of The Opposition to do the same?
We get this periodically with students trying to 'rebel' by refusing to wear black tie to dinners which require it, or other dress infractions just really for the sake of it. It always smacks of immaturity, and an inability to not draw attention to yourself.