You have to help her understand that the HULK is fictional and fun to pretend to be, but you still have to be responsible. BRUCE (Sorry Gamma G*) Banner is a mild mannered person who "Hulks Out" from time to time does she understand WHY?
To quote Spiderman ... "With great power comes great responsibility." Maybe that needs to be introduced.
My son has been big into the superheroes since he was very young too, but one of the things we stressed was that they were FICTION.
So, I will suggest you find a safe time/space that it's ok for her to "Hulk Out" and when she is not doing it appropriately talk to her about it.
EX:
LO: GRRRR HULK SMASH!!!
You: We are at the grocery store, we do not Hulk Out at the grocery store, but you can when we get home - time to be "Little One" again.
If she does not comply then a consequence. This way she can be the Hulk but there is now an appropriate way to do it.
*His first name was only "David" in the old TV series, in the comics and anything based on the comics he is referred to as Bruce Banner