Do you want to feel out of place?
I'm a country girl and the dining room is like going to a classy restaurant. The dining room is elegant with it's chandaliers, white table clothes and silverware. I wear my jeans when I eat at the buffet on the Lido deck, but you wouldn't catch me wearing jeans in the dining room.
While on a cruise I sat at a table next to a group who wore jeans in the dining room; they were informed that for the second night they were required to dress accordingly. They were told in no uncertain terms do NOT wear jeans in the dining room. I felt embarassed for them.