Out of curiosity, where did you get these numbers from? I’m not from the US, but what I do read and see on the internet is that people in the US also have a lot of debt (education, medical, and of course credit) I’m surprised that even for median disposable the US is number 1. But then again, maybe it’s a case of the internet being drowned out by a vocal minority?
Rich people have more debt than poor people because they have more ability to repay. The only way debt could truly signify one country is worse off than another is if the situation were unstable (people accumulating debt in a short-term crisis and they will not be able to repay).
I was also surprised, but it seems to be true. Wikipedia lists the median disposable income after adjusting for local prices (PPP), and USA is first on that list.