It's probably just the converter... something misconfigured in the drivers or... who knows. Try and see if you get the same garbled data in Windows and Linux (binary check). If they match, it's something hardware wise.
Back in the day, yes, the Chinese USB to Serial/Prallel converters were terrible, no doubt there. But, over the years, they've gotten surprisingly good. In fact, I picked one up a few weeks ago (as you said, about $5), works like charm.