I feel like Ranald would probably have said something if he cared about Mathilde reading the forbidden book, and that he probably didn't because he's a god of theft and deception, and we certainly are using stolen knowledge that we most definitely cannot tell anyone about.
But I do think he'd be worried if she actually used necromancy, because most of what necromancy actually does is against Ranald's strictures. Ranald is a god of freedom, and necromancy can enslave people's souls after their deaths. Ranald is a god of cunning, and necromancy (all Dhar uses, really) is universally used as brute force than any real elegance.
But going ahead and reading the thing people would most definitely persecute her for, as long as she's not actually hurting anyone? That's within his sphere.