In the U.S. today, that rude remark is likely to be true, for the same reasons that "whenever I see someone wearing socks, I assume they're seriously in debt" or "whenever I see someone with hair, I assume they're seriously in debt": if most people are in debt, then most fat, thin, bald, hairy, pants-wearing, skirt-wearing, or naked people will be in debt, because none of those attributes magically protects someone from being in debt.

The better question is, in the U.S. today, why does this person think "seriously in debt" is anything like a character flaw? It isn't even a neutral aspect of personality (such as liking movies or preferring blue pens to green), any more than "having been born after 1930" is an aspect of character rather than a statistical likelihood for a randomly selected person walking around an American city in 2013.