For me, it's a matter of degree. If it had been a realistic length for a year's natural growth -- about 12-15 centimeters, say shoulder-length, about as long as mine -- then it would be self-explanatory. She's been busy searching for Discovery and for answers about the Burn, so she hasn't bothered to cut her hair. As a ponytail-wearer myself out of expediency, I understand that perfectly. But typically for DSC, they took it way, way overboard and had it clear down to her waist. Which has to be an artificial indulgence, an extravagance that seems inconsistent with what she's been going through in the past year, in very much the same way as Daisy Johnson's sexed-up blonde-and-purple look in AoS season 6 (also after a year-long time jump spent wandering deep space in a relatively small craft). It just doesn't make sense to me, unless there's some story behind it that we haven't been told.