I agree with you Franbern.
For over twenty years I have housed vulnerable young adults in my own home, through an homeless charity, until they are housed.
Part of the agreement they sign is that they will be appropriately dressed during the day, unless in their room or bathroom.
It is better for mental health to get dressed, unless you are ill, even if only a tee shirt and jogging bottoms and we are asked to encourage our supported lodgers to go out of the house most days, even just a short walk, bus ride or trip to the supermarket, to acquaint themselves with the area.
How I would word it Peacelily is to just plainly say that you’re sure she won’t mind you mentioning it but you would like her to be dressed if she comes down stairs during the day, apart from early morning or evening. You don’t need to apologise or give her a reason, just a plain statement.