Premise: Thimble is cursed to speak only the truth. This isn't the sam thing as an inability to lie. That could be done on a purely psyhological level. Her curse was intended to check anything she said against the actual state of the universe. This has to have ben done with Systemic magic; blood-magic doesn't have the finesse. At least with respect to local conditions, it could have been done by any sufficiently creative and determined Systemic mage. (Systemic magic doesn't extend past the thermosphere, but as of the time she was cursed, the tech to leave it didn't exist, and any messages entering could be modified.) What actually happened, due to the equivlent of a syntax error, was that the world gets changed so that it matches whatever she says.
Obvious question who cursed her? Many mages could have done what was intended, but what happened needed the Systemic equivalent of root access.
A Matagot could have done it, but probably not by mistake. The Matagot work by necessity without a lot of the system's built-in safeguards. They know it very well and have incredible control. A Matagot can burn out or set off a chain reaction, but only on purpose.
Who else has root access? Even gods don't; gods work on a third type of magic. The founders of the system did, but most of them died starting it up. Only the four Deaths hung around.
One of the Deaths would be able to pull off a curse that affects the whole world, and do it without safeguards. They don't make new spells often, so they might well mess it up. And they wouldn't be likely to notice the subtle effects at first, thanks to the static of the System as a whole.
Which leaves the question, what did her parents do to annoy Death so badly? It's not like they were Systemic mages themselves; I can't even figure out how they'd meet.
On the other hand, now I know what to do with Thimble. She didn't geas herself to silence while she worked for Lawn; she did it by determination alone, proving she has tremendous self-control. Thimble isn't her real name, and almost everyone who knew her real name is dead. And she likes peace and quiet and keeping things in order. Thus, should one of the four Deaths, oh, retire in shame after having accidentally cursed someone in a way that might have broken the System and ruined the world, she'd make an excellent replacement.