Obviously we don’t know for sure, but it seems like Steph doesn’t have it in her to be that mean, conniving and self centered. I think she looked beaten down until Charles came along, and I hold out hope that she’ll get it together eventually.
I think Steph just can't get out of her shelf. It's more than not caring (also it has gotten better since Charles' birth).
I'll have to guess here: But to me Steph has this - all is useless attitude.
Like: Why should I do anything (wash my hair, wear appropriate clothes), when nothing changes? (I mean in regards of her fil and mil). I think Steph is intelligent enough to know that something is deeply wrong, but she's not the type for fighting.
I mean shortly after Charles' birth it seemed like she had gotten it...now she's back to wear it started (although with a smile on her face) - there's more wrong in Luxembourg than the clothes of the Hereditary Grand Duchess.
To be honest, Steph was a bad dresser even before the engagement.
And her all is useless attitude never occurred when she met other royals, then she always managed to doll up, at least by her standards. Strange, isn't it?
MT is out of Luxemburg most of the time and thus shouldn't bother her much. But whatever trouble there might be within the family, out of shear respect for the ("ordinary") people she meets, she should do the most basic grooming. If she was a normal employee and would represent her company at a fair in a sloppy outift, she would be in serious trouble, family problems or not.
Masako certainly had a hell of a time for many years through the imperial household (plus Michiko obviously wasn't a supportive MIL), but when she appeared in public, she always looked neat and appropriate.