Personally I do think that flight crews should do their best to look smart and professional at all times. Having seen some of the scruffy looking youths who fly for a certain airline from STN (not Go or buzz), if that's the sort of oik who flies for them then I certainly don't want to be a passenger on their airline!
As someone else has said - the hard working cabin crew are expected to look their best at all times and the flight crew should do the same.