I agree with everything you say and I am not trying to take anything from Buggy as far as his threat level goes. He's clearly seen as a huge threat. It's just that he wouldn't have any influence over Mihawk's bounty the same way a Yonko captain would have an influence on their subordinates. Take King, for example, they fear King because they'll have to deal with Kaido if they did something to King, which means Kaido has an influence on King's bounty.
When it comes to Buggy and Mihawk, they don't fear Mihawk because they'll have to deal with Buggy; they fear Buggy because they'll have to deal with Mihawk. So Mihawk is the one influencing Buggy's bounty, not the other way around.