I partly agree with Sonny here. Don is originally a term of respect. It comes from the Latin word Dominus (used in old Rome). And that had nothing to do with crime bosses.

But...

As Olivant stated; in the GF movies it's used as a title for a crime boss. To me it's more logic that because of the use of the word Don for the mayor, it's clear that he's also a crime boss. Even if he didn't look like one. Probably we can't clarify that part.

Isn't there anyone else called Don where it's more likely he isn't a crime boss? If that's true I give Sonny's arguments the advantage.