Jeans on Carnival? Yup, you won't be stared at. I consider Carnival one the most casual lines.
I belive others brought up some very sensible and valid points. It is too hot for jeans, and they take up too much space in luggage.
I do think white jeans are a step up, they can look quite nice with a nice top, in fact they actually just look like casual white slacks , which are fine for the dining room on any night but formal night.
I do think jeans on formal night is tacky. Everyone have something they can wear that is comfy and anything would be better than jeans.A pair of slacks and a blouse would be fine, ( although I personally dress for formal night) . I think anyone who wore jeans to formal night( and had in fact NOT lost their luggage) is being a bit rude and self centred.
I understand not wanting formal wear, but there are a lot of choices between jeans and formal wear, and if you can afford a cruise you can afford a pair of slacks or a skirt( I found some great outfits at thrift stores for under 15 bucks)