I dont think it is, and it wouldnt make much sense even if thats the case. Bounties are not a powerscale, it is simply a way to measure how much the government wants someone dead and/or captured...Robin had a 80million bounty as a kid, but its pretty obvious that 8yo Robin was no match for east blue Luffy, but he was just worty 30M....
Shanks is Mihawk rival, but unlike Mihawk he has a crew to back him up...By fucking with Shanks, you fuck with his crew, that plays a big role in his bounty. Mihawk on another hand dosent have a crew.