After a bit more thought, here's my thesis.
Jodie Whitaker's costume has been designed to soften the blow to the less progressive section of the audience of having a woman Doctor in the first place.
First, I think there is a sexualisation aspect, though it's softened by the rest of the outfit. Dan may joke about my "exposed clavicles" but there's a reason so many ball gowns left the upper chest and area between the shoulders bare while covering everything else. It's a highly erotic area, being very sensitive to touch and also the part to which a lover's kisses turn when they move from the mouth and face. (Note, I'm not suggesting this aspect of her costume is designed to provoke this association, I'd just argue that it's there in the background.)
It also makes her look vulnerable, and emphasises her relative youth, which I now think is more important. The closest we've come to a comparison for her costume is Mork, a character who is innocent and pretty much an adult child. On top of that, the 13th Doctor's overall look is that of a student.
Note that all her previous, male, regenerations have dressed like professors or teachers. Matt Smith, an actor in his twenties, wore clothes that made him seem more like an academic of forty or fifty. After twelve male regenerations dressing like professors, we have one female one dressing like a student.
So rather than making her seem mature, eccentric and mysterious, Jodie Whitaker's costume is meant to make her seem young, kookie and non-threatening, the last a quality only Peter Davidson comes close to matching.
And who might find a female Doctor threatening if she was forty or fifty and behaved in the same irascible, domineering way her previous regeneration did? I'd argue a large part of the male audience the BBC cannot afford to have switching off, especially worldwide.
So maybe this is a necessary first step to full equality. But I fear it won't prove to be full equality yet.