Maybe he doesn't in so many words, I thought he did but...
but I thought it would be obvious to people, The Encyclopedia of Arda says that if she had become Queen she probably would have lifted the Numenoreans from their darkness and I can't imagine Tolkien intended Miriel to be an evil character
The reason I'd always assumed that she went too late to the top of the holy mountain is that she was seeing the entire kingdom drowing in front of her eyes and that might of distracted her from her own personal safety.