Below is the Carnival dress code from their website and also in the Welcome Aboard book that comes with the documents. I do not see where it states a No Jeans policy in the dining room ( I don't wear them, and it does not bother me if someone else does). How can a cruiseline be expected to enforce a policy they don't have in place?
As with most dress codes, expecially one like Casual Attire, the interpretation is relative to where you live, your upbringing, your job, etc... At the end, it gives a sampling of casual resort wear, but it still does not mention specifically that Jeans are not permitted. Seriously now, are you really going to get upset if someone comes to dinner in nice Jeans and a Polo type shirt on non formal nights?
Casual attire and resort wear is the order of the day both on board and in port. Shorts, slacks, sundresses, blouses, etc., are in line for women. Clothing for men is just as casual. You should bring a pair of rubber-soled sandals or sneakers to wear on deck and a pair of good walking shoes. You might also want to consider bringing along a sweater or jacket for cool evenings and inexpensive rain gear in case of a sudden shower. Women may want to bring along a hat or scarf. You'll have a chance to dress up on two nights during 7 day or longer cruises (one night on shorter cruises) for the Captain's Cocktail Party and the Gala Farewell Dinner. On the other evenings you can dress casually for dinner (no shorts or tank tops).
For those not wanting to dress up, the Lido Restaurants are open nightly and have a casual dress code. Formal Wear: tuxedo; suit and tie; sport coat, tie and slacks; evening gown; cocktail dress; pantsuit Casual Resort Wear: sport shirts and slacks; dresses; skirts; pantsuit; Capri pants NOTE: Shorts, t-shirts, tank tops and bathing attire are not permitted in the Dining Room during dinner.