She is queen because she ‘made’ herself queen. Even if there’s a baratheon somewhere that can claim their rightfull place on the Iron Throne they probably don’t have the support or the army to ceased it from her. It’s a mute point now. She killed everyone to become queen.
Especially with Renly and Stannis getting their armies decimated fighting.
Just wait til Gendry rows into Blackwater Bay with an army of mermen, he’ll show her what fer.