As TV Tropes explained:
"To show that a character is in such dire financial straits that he's literally "lost his shirt" (though there are times where a character has to wear a barrel because he or she lost their clothes, not because he or she is poor) the otherwise naked character will resort to wearing a large barrel held up with suspenders. Primarily seen in cartoons. This image probably came from a punishment for public drunkenness in Germany and England, where drunks had to wear a booze barrel. It's unclear how barrel wearing became associated with bankruptcy, but the trope stuck."

Victini: You need some assistance with something? Judging from your condition you seem to be in trouble with something..

Lucario: *Why does everymon assume I'm broke, this is a fashion statement in this area.*