I don’t really ever leave my house and I live in loungewear. I ain’t changing just to go to the store. That’s a ridiculous waste of time and energy. I don’t think that most Americans care what other people think about their clothes.
I have seen this on very few occasions, and each time, the pajama-wearing individual is very obviously only out in public so they can either stock up at the liquor store or meet their meth dealer. I don't think this is common.
I had coworkers in the early 2000s who would do this, working in a white collar profession, and pretty sure they weren't alcoholics or doing (hard) drugs.