I personally believe that i makes PERFECT SENSE.
I have beem going through all the chapters trying to justify mihawks bounty. And your reasoning is the only plausable one.
He doesnt have a crew, no subordinates, no aspirations of over throwing the world governmemt, no underground network, no affiliation with a formee powerful pirate crew. Plus his bounty has been frozen until now
So he had to have done something so crazy in the past that he got his title with the world goverment being scared of him since he was destroying their military power