I'm very casual and comfortable for therapy (jeans and a t-shirt) because I don't do much else these days (certainly not anything requiring dressing up). The one time I wore a button-up, T looked a bit shocked until I explained that I had not done laundry in way too long and had run out of clean clothes. I think my T would be really worried if I ever showed up in sweat pants or anything more "dressed-down" than I currently do because I hate wearing anything other than jeans. If I'm in something different (pj pants, sweat pants, yoga pants, anything loose and without a button and pockets), I'm really not functioning and it's a problem.
In the past, I would show up to therapy after work or school. Then my dress changed by the requirements of where I was prior to my appointment.
I would think T's note any changes in attire/appearance as it can sometimes be used to help determine functioning. Even if I don't shower all week, I will shower before therapy because 1) gotta keep up appearances, 2) I wouldn't want to tortue my T with a stinky, gross me for an hour in such a small enclosed space.
I've never felt the need to dress up when going to a doctor or healthcare provider, however I will make it a point to shower if I'm not really ill with something. Of course, I never really dress up for anything anyway...