Tyrant: a cruel and oppressive ruler.
Cersei is prettyextremely cruel, I don't think we really need to explain this or argue against it. Remember that she also tortures Septa Unella and Elia and Tyene Sand.
Oppressive: inflicting harsh and authoritarian treatment.
She very much does this, she straight up murders all of those who oppose her, as you say with the Sept of Baelor "incident".
And using the common people as human shields is a very obvious and cruel thing tot do, I don't think anyone would deny that.
As xdtTransform mentions in a comment it's also worth noting that taxes probably were raised as Cersei has a lot of bodies in King's Landing at present that need feeding and that food needs to come from somewhere and you can guarantee the common folk will be the last to receive it.
Being a tyrant doesn't necessarily mean she is doing harm in her rule or inflicting damage on those under it. However, Cersei doesn't care about the people of the King's Landing and solely about her children and maybe Jaime. She wants power for the sake of power not to make the world a better place.