I think calling yourself emperor was all the rage at the time.

Prior to Napoleon and the dissolution of the HRE the Roman Empire was sort of still viewed as single and indivisible, it wasn’t a exactly a generic title (well.. at most there could be two legitimate ones).

But now you had the Emperor of the French, Emperor of Austria, Russia (prior to that), so why not Brazil? Since you’re establishing a new state anyway might as well pick the best available title.