Right…so this is my thinking *as a non book reader*
Technically Joffrey can't because he's 1) illegitimate 2) not the oldest b*stard. If he were to make that decree, he would be admitting that from the moment his Father died, until the exact moment of his decree, Stannis was the rightful king. In essence, if Robert knew Joffrey wasn't his son, and he made the decree before he died, this war would be quite pointless.
With that said, as it stands now, technically Kelly C is the queen regent right? But because she can't have kids, the Targ blood line is officially dead. She's only a stand in, until she dies. So no matter what, if anybody wants to "rightfully" own the throne, both kelly c and stannis and stannis' daughter have to die. Once that's done, the baratheon name has no rightful heir and the throne is completely up for grabs. Right?