This question already has an answer here:
In season 6 finale Cersei crowns herself queen of the Seven Kingdoms, but whom will she de facto rule over? Even if we put aside that her realm is about to be attacked by two great forces from the north and the east, there doesn’t seem to be much support for her Iron Throne in the Seven Kingdoms. The North has essentially seceded, as have the Iron Islands, while Highgarden and Dorne are plotting against her. The Vale seems to have aligned them selves with the Starks and while the situation in The Stormlands is currently unknown it is unlikely that they would support a Lannister queen. This leaves only The Westerlands (which were “hers” already), The Riverlands (which might be in disarray after Arya killed the key Freys) and The Crownlands.
I realize that this probably doesn’t even matter in the grand scheme of things with the eminent arrival of the Night’s King and Daenerys, but it just seems that she will not be ruling much, something that she is probably oblivious to in her blind quest for power.