Dany has spent the entire series learning to be a proper ruler and essentially going through a checklist of different Targaryen personality flaws and learning to do the opposite. She isn't perfect by any means but she is probably the best option outside Jon Snow who doesn't want it or Tyrion who would never be allowed to have it.
No she hasn't. She has spent the entire series being petulant time and time again, reminding people of her supposed birthright time and time again, using her dragons to intimidate time and time again, shaming people who try to help her time and time again, and did I mention doing dumb shit time and time again? She has shown zero traits befitting of a ruler.
Cersei, for all her cruelty, ascended to the throne and knows how to be a politician and tactician. She is not fit to rule, but neither is Danerys, by all sensible standards. Then again, I'm not sure the 7 Kingdoms ever had a good regent.