Doflamingo wasn't particularly afraid of Aokiji. If fact he broke out of the ice which no one but Whitebeard has shown to do (even then, Whitebeard didn't do it as quickly). Plus he had the likes of a warlord (Law) and Vice Admiral (Vergo) and presumably 3 others in Law's power range under his authority. He's probably about Admiral level. The reason he left Aokiji was because he needed to hurry and get Caesar and take down Law. Even Akainu took 10 days to barely beat Aokiji. Whether or not Don could beat him he didn't have that kind of time.