I don't know whether the statements by Daenerys and others in the televison series Game of Thrones are based on statements in the novel series A Song of Ice and Fire that Cersei is a tyrant. I don't know which writer(s) are responsible for those statements and what they meant by them.
But it is possible that the writer or writers who describe Cersei as a tyrant might be aware of the history of the word tyrant.
The English noun tyrant appears in Middle English use, via Old French, from the 1290s. The word derives from Latin tyrannus, meaning "illegitimate ruler", and this in turn from the Greek τύραννος tyrannos "monarch, ruler of a polis"; tyrannos in its turn has a Pre-Greek origin, perhaps from Lydian. The final -t arises in Old French by association with the present participles in -ant.
"The word 'tyranny' is used with many meanings, not only by the Greeks, but throughout the tradition of the great books." The Oxford English Dictionary offers alternative definitions: a ruler, an illegitimate ruler (a usurper), an absolute ruler (despot) or an oppressive, unjust or cruel ruler. The term is usually applied to vicious autocrats who achieve their goals by unjust and harsh means. The definition of a tyrant is cursed by subjectivity. Oppression, injustice and cruelty do not have standardized measurements or thresholds.
in ancient Greece, tyrants were influential opportunists that came to power by securing the support of different factions of a deme. The word tyrannos, possibly pre-Greek, Pelasgian or eastern in origin, then carried no ethical censure; it simply referred to anyone, good or bad, who obtained executive power in a polis by unconventional means. Support for the tyrants came from the growing middle class and from the peasants who had no land or were in debt to the wealthy landowners. It is true that they had no legal right to rule, but the people preferred them over kings or the aristocracy.
So by the original Greek definition, the leaders of the French Revolution were tyrants, no matter how good or how evil one thinks that they were. And the founding fathers of the USA were tyrants by that definition.
In late antiquity the word tyrant meant an Roman usurper who claimed to be emperor but didn't succeed enough to go on the lists of emperors.
For example, the reign of Gallienus (260-268) was full of attempts to usurp the throne, though the claim that there were thirty tyrants during his reign is a bit exaggerated.
Famous successful Roman usurpers or tyrants by that definition include Vespasian, Septimius Severus, Diocletian, St. Constantine I the Great (the first Christian emperor) etc.
Roman usurpers or tyrants in Britain or associated with Britain included Carausius (286-292, Allectus (293-296), Magnentius (350-353), Magnus Maximus (383-388), Marcus (406-407), Gratian (407), and Constantine III (407-411). It is no wonder that Gildas writing sometime about 480 to 580 wrote that Britain was a province fertile in tyrants. Gildas also described the invitation to the Saxon mercenaries as coming from the entire council and "the proud tyrant" implying that there was at least one self proclaimed emperor in post Roman Britain. Procopius also wrote that since Constantine III to his own time Britain had been ruled by tyrants, implying that there was a whole line of emperors in post Roman Britain.
So it seems possible that when Daenerys said Cersei was a tyrant, she didn't necessarily mean that Cersei ruled in a harsh and cruel manner, but possibly meant that Cersei had illegally usurped the rule from people with a better right to rule, like for example Daenerys herself in her own opinion.