DD13 was like this. She was diagnosed with ASD a few years ago, to the utter surprise of her teachers, but the complete non-surprise to those of us who live with her when she doesn't need to mask!
Just like you, I couldn't understand how she could spend ages obsessing over make-up, yet didn't brush her hair (it was seriously horrific at times, a literal matted bush), and would forget to to go to the toilet (too absorbed in whatever she was into, then she'd have accidents) and never brushed her teeth. She needed the hygienist at one point and even that didn't seem to sort it.
I was really worried about how her lack of personal care would affect her at secondary. As it turns out, the anxiety over needing to fit into a new school (I assume) somehow got through to her and almost over the summer holidays before she started Y7, she got it together.
She could probably shower a bit more (not that she is smelly, so I'm reluctant to push it) but she now takes fantastic care of her very long, thick, curly hair, has a brace and her dentist says she's doing a fantastic job of keeping her teeth clean, and hasn't wet herself in years as far as we're aware now. She's also less interested in make-up..!
I did provide her with her own decent hairbrushes, fancy electric toothbrushes etc which made little difference until she actually wanted to change, and then she was presumably grateful she had those things.
She does struggle with anxiety and sometimes depression I think - it is hard to tell as she is a very private person, although sometimes it will just all spill out if we get lucky. I always make sure she knows I am there for her, but you know how it is.
Depression won't help though. If she's struggling with friendships in particular, it can feel very hard to climb the mountain to "fit in" and perhaps as a result she can't see the point in trying. I do think having a steady friend during the transition to secondary helped DD a lot.
I don't know what the solution was for DD. I'd assume it has to come from within, they need their own internal motivation - I'm going to guess for her it was the very common female autistic need to blend in, and realising she'd need to do more at secondary than at primary where she was relatively content and accepted. Or maybe her appearance became her special interest, who knows? I'm just very relieved.
Good luck!