Maybe no jeans and t-shirt, but very casual and unpolished, with hair in a schoolgirl style. That is what gives one the impression that she wears what is easiest fo find in her size.
The tin man suit, the pregnant trench coat, dress with sequined epaulets, see through blouses, fur trimmed vest are choices that didn't quite make the princess level.
Another commoner who married into a royal family and has carried off the style of her new position is Maria Teresa of Luxembourg. She always looks polished, though sometimes wears clothing that would look a little better on a slimmer person. Marie can step it up a bit, as she has the figure to make a stunning princess.
Her formal attire has been much better, such as her wedding gown and the blue dress for the new year's event, but the two purple dresses fell a bit short of the mark.