My first thought was that many people who are depressed find it very difficult to get motivated to do things such as getting dressed and brushing their hair in the morning. While it is probably only a small minority of those out and about in PJs, it could be hard to know who is in this situation - but making comments, shaking our head or taking photos could make that person feel a lot worse.
I don't think taking photos and publishing them on FB or elsewhere is acceptable at all and would it extend to those in bikinis, low-slung jeans or any other style we personally found a bit odd/unattractive ....? There are many looks which some of us clearly find less appealing than PJs and dressing gowns.
On the other hand, taking the time to get dressed to go out as well as choosing the type of clothes we wear (eg colourful, smart etc)can have a positive effect on how we feel.
There is some interesting research on "enclothed cognition" relating to this.