The TV series Mad Men plot starts in 1960, where we can see that hairstyle is mostly short, greased, combed, flat and facial hair style is freshly shaven.
Matthew Weiner seems to have worked close to mid/late 60s reality and trends for his characters and we can notice most of them adopting various trends that were growing popular back then. For facial hair: big beards, whiskers, moustache... For hair: ungreased, uncombed, mid-long.
However, Donald Draper stays always the same and keeps his hair flat, greased and combed. I think that this is meaningful for the character development.