This is obviously a hot button issue, but the truth is, while not recommended, jeans are tolerated in the dining room on Carnival. You won't feel out of place, no one will give you dirty looks, and you won't have any problems. Been there done that.
In fact, similar to Jean S., when we informed the waiter that we wouldn't be attending formal dinner due to my lack of formal wear, he insisted, firmly, that we attend regardless, and I'm glad we did. Formal nights in the dining room are the hightlight of the cruise. I did have a pair of dressier slacks along that I wore on those nights, but never felt out of place in jeans on the other nights either. Like it or not, the "dress code" on Carnival is not an issue once on board.
Enjoy your vacation, and everyone quit worrying about what everyone else is wearing!