Mentioning a previous answer is not intended to discourage further questions. More questions, information, and debate is always welcome.
'"[J. F.] Kennedy despised how he looked in hats and usually refused to wear them - despite howls of protest from habderdashers whose hat sales took a precipitous downturn in the 1960s" (Barbara Perry). What factors led to the downturn in menswear hat sales in the 1960s?' at https://www.reddit.com/r/AskHistorians/comments/68hta6/j_f_kennedy_despised_how_he_looked_in_hats_and/ has several answers by /u/chocolatepot
There is a question "In the early/mid 20th century did men really wear hats all the time, and if so, did they always have crazy hat hair?" with discussion https://www.reddit.com/r/AskHistorians/comments/469icw/in_the_earlymid_20th_century_did_men_really_wear/d03j32x/ The links on the uppermost reply were not curated, so cum grano salis. There is some support to the "because college got informal" thesis, but other factors were cited in surveys from the time frame.
Two things that should be debunked: that hat use fell off a cliff in the 1960s, and that John F. Kennedy was to blame. Hat use had been declining for years, and Eisenhower may have been hatless a lot (I don't have a good source yet).