The craziest thing about this episode, more so than the mustaches, is Don wearing shirts to work that aren't plain white. He's literally never done that. Even his casual outfits, he almost always still wore a white shirt under his blazer. Even Don Draper can't resist the tide of 60s fashion forever...
As much as anything else it's a clever bit of period detailing; even men who (very wisely) decided that late 60s/1970s fashion was
awful - and so continued to dress like it was still 1966 for the rest of their lives - generally had no issue with wearing coloured or patterned shirts.
