My impression of Mel is that she has power but is not all-powerful. So... I don't know.

Yeah, that was kind of what I meant. What she did with Renly could have been dismissed as just her doing it. If that power (and even more of it, since she seems truly shocked that someone can be brought back that many times) isn't exclusive to her but is actually something that others can use too, she becomes a more interesting character IMO.