Definitely not an urban myth on Carnival! Problem is they have the cheapest rates, which leads to a cheaper crowd, which leads to a more party-type crowd. For the most part, younger people don't have the money, so if they want to cruise, they go for the cheapest rates. Younger people tend to party more than others.
Don't mean to sound like I'm stereotyping, but it's just the facts based on doing this for many years.
Of course, there are exceptions; for example, if you do a 7-night cruise when school is in session, you'll have less party-types. But if you do a 3-night cruise over the weekend, you'll definitely have more party-types.
We've been on Carnival cruises where there were hardly any kids on board and never had any problems at all. It was basically full of seniors, like us, so it was much more sedate. Yet we've also been on Carnival cruises where we just wanted to lock ourselves in our cabin because of all the loud and boisterous behavior going on. And forget trying to find a spot by the pool, not that we wanted to because of the noise.
As I said, depending on your likes and dislikes, what you choose will always have a major impact on what you get; the cruise line, the ship, the itinerary, and the date will dictate the make-up of the crowd, which will impact the overall experience.