As for victim blaming... Why?
My first thought was how uncomfortable it would make a lot of vulnerable SUs feel. Was your first thought that she would be the one who'd end up uncomfortable then?
In my previous post I was contrasting my professional environment with how I feel as SU, because there is a power differential in the latter. And that makes a big difference to the impact of how you - the professional with power over the SU - dresses, if how you've chosen to dress is with your underwear on display. Not to mention the fact that presumably she is also dealing with people who are already distressed or talking about/working through causes of distress.
It's intimidating, inappropriate, triggering, distracting, uncomfortable and distressing to be vulnerable, potentially dealing with difficult things, and then to be faced with a person who has power over you deciding wilfully to display their underwear to you. It has the potential to erode trust and make people feel unsafe.
I wouldn't care about a bra strap. I don't want to see the knickers or suspenders of staff treating or supporting me. I would be upset if they were clueless enough to need that explaining. I suppose if she doesn't understand this though she might also lack the insight to understand the power imbalance that exists in her role?
In my own workplace the power differential is only in terms of seniority. It's nothing compared to that in mental health settings. We're peers at work, albeit with reporting lines, so the impact of clothes choices doesn't have the same impact on people.
I wouldn't appreciate a colleague exposing her knickers/suspenders to me, but it also wouldn't make me feel intimidated or vulnerable in the way it would if a mental health professional did that to me.