Robert Baratheon became king in part by right of conquest but also because he had the best claim to the throne (not counting Viserys and Daenerys), because his paternal grandmother was a Targaryen. That was how they justified it.
Cersei has no claim to the throne and Westerosi laws of succession (outside of Dorne) favor males over females. So, theoretically, Jamie would have a better claim to the throne than she does.
Cersei was married to the king and mother to two kings, but she doesn’t have a claim on her own. But now she’s killed off most of her opposition (or so she thinks), so there’s no one left to object.
“Cersei has no claim to the throne and Westerosi laws of succession (outside of Dorne) favor males over females.”
Cersei has a claim as wife of Robert Baratheon. The laws favor males over females, certainly, but they don’t preclude females if there are no males left to inherit (just look at Lady Lyanna Mormont, a nine year old girl who inherited the lordship of her house).
“So, theoretically, Jamie would have a better claim to the throne than she does.”
Jaime is a knight of the kingsguard so he forsook any claims he might have had, which would have been pretty weak to begin with, as a brother-in-law on the female side.