Why are Cersei’s kids considered “abominations”? Tommen and Myrcella are children who have done absolutely nothing wrong, and even Joffrey can’t be blamed for his conception. Is there a historical parallel here or is this an example of GRRM making use of values dissonance?

I think what you’re missing here is the connotation of “abomination,” which is religious in origin, whether we’re taking the original Latin meaning of an ill omen or how the word was used to translate from the Hebrew word “sheqets” which means ritually unclean or forbidden, taboo.

From this perspective, it doesn’t matter what Joffrey, Tommen, and Myrcella have done, it’s not a question of their culpability for an action, because in their very existence they are a violation of the laws of the Old Gods and the New. Hence why in the books the High Sparrow refused to give his blessing to Tommen without being “convinced” that the story isn’t true, because to do otherwise would be to taint the spiritual purity of the Sept of Baelor.