This question makes little sense. The only definition of "successor state" that is relevant from the historical point of view is a concept where a newly established country "inherits" the relations of a country that no longer exists which might mean transfer of obligations, treaties, rights etc. You can find an overview of the concept on Wikipedia. A claim of succession generally relies on other countries to accept it. A typical example would be the Russian Federation which is the accepted successor state of the USSR. Modern Germany is the successor of the Weimar Republic which was the successor of the German Empire.
Brazil as a successor of Portugal makes little sense in this concept:
- It would require that the "original" Portugal ceased to exist at some point.
- It would require Brazil to be established after the "original" Portugal disappears from the world arena.
- It would require some action by Brazilian politicians to actually claim succession.
The first two points can only be fulfilled if you view the Brazilian war of independence as the point where the "original" Portugal ceased to exist and was split into Brazil and the modern Portugal. You probably recognize this as a pretty ridiculous point of view and I would be very surprised if the Brazilian politicians saw it differently.
What you probably mean is "Do Brazilians think that they are worthier successors to Portuguese Empire than Portugal?" This would be an interesting question but I don't think that it is a history question. Or maybe: "Which country has more influence today, Brazil or Portugal?" An interesting question as well but even less related to history.