Denim: It is what the industry has rendered ubiquitous, and more synonymous with the cheaper, fast fashion trends of youth - more units of jeans and tshirts are shifted repetitively, whereas dress purchases remain strong and constant, just not as repetitive. Basically, women will overbuy jeans and tees but not dresses.
it is far more difficult to engineer a well made dress than to simply 'block cut' a tshirt or boxy top. It involves working with the human form in a more direct way. Whether that dress is fluid, fitted or loose. I think we see far more wide and voluminous dresses now because the manufacturers dont have to spend as much on the designs or skilled cutters, since they simply scale up or down for fit instead of having to refit sleeves, waists, zips, etc. The one size fits all designs are cheaper to create and encourage less returns. Sadly it is limiting and most stores now look exactly the same.
Dresses are not inherently more dressed up than jeans. It is all about perception and what we grow used to seeing around us. Everyone defends the current shapes, generally, no matter what was said in the past.
If more brands created solid colour or non lurid-patterned dresses that worked 'with' instead of against the body, with less bizarre features such as bows, open backs and balloon sleeves, you might see them selling much more. Of course anyone can sit and relax in a meeting or interview in a skirt, just like with any garment, it depends upon the length and shape.
There is no such thing as a 'dress lady'. That sounds like something a toddler would say about an eccentric aunt with a uniform who might style herself on quirks such Lucy Worsley. And for Ms Worsley, her style is her 'brand', and does not represent how most women buy or wear dresses. She makes herself recognisable via her outfits, which would still be as unique if she swapped the frocks for trousers.
I have noticed, and this is to be quite fair, that people who make childish put-downs about timeless or non trend clothing are often insecure, or struggling with their self image, generally. The more relaxed you are with your body and style, the less you even care about other people's.
Dresses don't have to be office ready, 'dressy' or too feminine. They are simply a column or cut of fabric that is shaped for different styles over time that doesn't have a gusset. That's that.
It is pretty simple really. Only in a market oversaturated by box-like, masculine cuts would a standard a-line dress or a knitted wool dress look out of place. I can find pure silk as-new Brora skirts on ebay now because that look isn't super on trend. It all depends upon what you prefer.