During any of my 14 Carnival cruises, I have yet to see jeans in the dining room - on men or women. (Of course, I don't check each person's attire.)
However, I have seen a couple of Maitre d' doing their job properly by turning away people who are not correctly dressed. Good for them!
The dress code does not specifically say 'no jeans' (I wish it did.) The only thing that is in print is that shorts, tees or tank tops, or swimwear and coverups are NOT allowed in the dining room during the dinner times.
"Resort casual" is the proper attire for evenings on non-formal nights. To me, this is something light and airy, tropical, and that just doesn't bring to mind denim. Look at all the people in the TV commercials about cruising or island resorts - NO JEANS!