I do think as a society we are generally more casual now and I think there is an demise in many types of formalities, not just in clothing. We are more "casual" in a multitude of ways. In the US, speech and manners seem far less formal among younger generations. Younger generations seem perfectly comfortable referring to teachers, parents, bosses and others in authority by first names only. When our parents were kids, not in a million years would they have referred to a friend's mother by the first name. In schools, cursive handwriting is no longer taught. Fewer people write formal handwritten "thank you" notes, instead they rely on a quick email to speak for them. Telephone calls and face to face meetings are replaced by texts and the like. Why send a "Happy Birthday" card when you can simply text it? The boundaries, structure and formalities that were integral in forming older generations are far less prevalent in modern society in the United States. Is this inherently good or bad? I suppose it depends who is answering the question.
For me personally, I never even wore jeans before I was in college. Of course that may be because I couldn't find any to fit my scrawny body and flat tush!
My grandmother's voice still runs in my head, so I often think dressing well is a sign not only of self respect, but also respect to those you are meeting. Personally, I also feel better when I dress well. That being said, I enjoy the freedom of being able to put on a pair of jeans and ballet flats from time to time. But NEVER will you catch me in athletic wear or sweats outside of the gym... well the sweats seem to have found their way into my lounging at home attire... but I swear... never on the other side of the front door!