It is absolutely NOT necessary to dress like a mess to get across any point. Indeed, you dressing like you normally do would actually reinforce the fact that you are a well organized, put-together person, and yet your DD's behaviour is beyond what you can handle on your own. IMO, dressing like a mess says, "I cannot handle anything - not even something as simple as putting myself together in the morning."
I have been through all sorts of evals for my DS, so I have been in your shoes. If anything, I found that the smarter I dressed, the more likely the dr.s and therapists were to be straightforward in their information, direct about their suggestions and their opinions on outcome, and the more willing they were to give me concrete steps I could take at home to work on things during the times or days he wasn't in therapy. They didn't fear I would break down if they told me things, and they assumed I would understand clinical terms so they didn't "dumb down" their language. All in all, if anything dressing well was a benefit.
That's not to say that dr.s and therapists do not give equally good service to those dressed in sweats. But we all make assumptions about people based on appearances (whether we admit it or not), and I preferred the assumptions people made when I dressed well.