The last time I cruised NCL jeans were not allowed in the dining room, but I'm guessing by now (maybe someone can confirm) that they are probably acceptable now.
However, shorts in the dining room in the evening would definitely be a no no. As far as I know Carnival is the only major cruise line at the moment that will allow shorts in the dining room on "casual" nights.
jeans are allowed in one of the MDRs on all NCL ships and in both on some ships. As for shorts, apparently they are now allowed as well. We will be on the Spirit in a couple of weeks, I will report back then. Not, that I like to see shorts in the dining room, but I guess pressure has caused the dress code to continue to become even more casual each year.