I might be a little biased but my opinions are only based upon the reports I read and hear (YouTube and news channels) about Carnival ships. i.e.- brawls breaking out, drunken people partying, people falling overboard, etc.
If it were only one or two occurrences over the years, I might tend to believe that Carnival couldn't be all that trashy but their reputation hasn't changed from many years ago.
If Princess and the other mass market cruise lines had similar travelers why don't I see and hear similar stories about them? I tend to believe it's the types of people that choose Carnival in the first place.
I guess I'm just judging the book by it's cover.