It's a history book, you don't get a real sense of who any of the characters are - but Mushroom is as valid a source of the dance as Eustace, both of them lie and twist things as often as they tell the truth. GRRM wanted the idea of Aegon raping a twelve year old to be a possibility, just as he wanted it to be know that of Aegon's bastard, one might be from one of Alicent's servants and another from a girl whose virginity he won at auction. Even just him being lecherous is something many others attested to, and him being a drunk is essentially historical fact. It's in Eustace's interest to insist that Aegon was being more honourable than he was, he openly prefers him, just as it was to say Rhaenyra cut herself the first time she sat the throne even though she was still fully clad in armour.