It's pretty clear these dragon fish are absurdly profitable, so it's not completely unreasonable. Also, the average strength he's encountered was referenced according to the cities he lived in at the time, not the country as a whole. I think
The only over-the-topness that comes to mind is the main hero's strange ability to attract all kinds of trouble and scumbags wherever he goes, but it would be pretty boring otherwise, I guess. I'm relieved he ditched that girl that he met early on, cause he already has trouble defending himself.