My narc parent does this. Dresses very scruffily at inappropriate times. Also very capable of looking immaculately turned out when it suits. I am no expert. Just pointing out a similarity I have noticed. For me it is ego driven - I don't have to make any effort because I am wonderful.
It's also 'I don't have to make any effort because you aren't worth it'. As per people such as the vaguely creepy Tory Boy I used to live near and was friends with his wife (who was, in all fairness, lovely) - everything that mattered to her, whether personal or work related, he showed up in board shorts, a creased Hawaiian shirt and flipflops, even in winter. But once he decided he wanted to become a Tory councillor, it was Armani suits all the way.
Or my mother, who turned up to her own son's funeral in a grey teddy bear fleece jacket and knackered black leggings, but for her trips to the doctor where people who mattered might see her
when trying to get another prescription for opiates it was a clean skirt, a usually fresh out the packet top, hair done and lipstick on.