He's not. He removed the Empire which included having to remove Empire supporting Jarls, as they would keep fighting otherwise. The fact that the Jarls will no doubt support him is simply an unforunate by product of the war. Elisif and Tullius do the exact same thing.
Actually not really, one of the new jarls could run for the office of High King and the imperials never go "They better vote for Elisif because our armies are in their cities" whereas Ulfric does say that.
He has a legitimate claim to the throne. By that reason he can't be a dictator.
Since any Jarl can run for High King each Jarl has a claim to the throne but Ulfric is making it impossible by having his armies in their holds and essentially threatening them.