Ace seemed to be under the impression that Whitebeard wanted to be PK, we know how that was in reality.
We need to hear more from Doffy himself to settle the matter.
Altough Donquixote pirates are drastically different from Whitebeard pirates. WBs care only about freedom, adventuring and dreams, while Ds are much more business-oriented. Running business is all about precision, not vague interprations about it's goals. I'm sure if Monet thought Doffy wanted to be Pirate King, he must have mentioned or at least hinted it at some point.
Tough I admit, such a romantic dream which ultimately means nothing practical expect finding a load of booty doesn't seem something Doffy would desire.