A fairly hefty price tag for a typical survivor deck (and Stella Clark out of the box doesn't have a strong economy engine), but Orne is a solid buy, especially for Miss Clark, whose "fail and go again" strategy benefits from controlling degrees of failure. While Granny's +1 helps you pass many Treachery tests, her ability reduces the effects of Rotting Remains, Grasping Hands, and so on and lets you you pick your poison on cards like Abyssal Reach. More positively, she also helps you trigger Grimm's Fairy Tales, Oops!, Dumb Luck, and "Look what I found!", etc. She's not as useful a Horror soak as Peter Sylvestre, but 3 Sanity is nothing to sneeze at. Granny Orne might find a home in a number of Survivor decks, and her upgrade is even better.
Tough Old Bird
You get +1 .
When an investigator at your location would fail a skill test, exhaust Granny Orne: That investigator fails by either 1 less or 1 more.
So, Granny Orne (0) is a decent L0 ally for Agnes. She comes with a +1 boost, unlike Peter Sylvestre (0), and while she can't soak horror quite as well as Peter, she still gives you a decent buffer on Agnes' precious sanity. Especially significant if you're working from a limited card pool.
I think it's also worth considering that Granny (0) is very helpful for allowing you to trigger Grimm's Fairy Tales, which is actually a pretty efficient horror healing card if you can get it to fire reliably. If you fail a test you are normally failing by 1 (which Granny can make 2), or you're failing by 2 or more.
I think there's a really interesting Agnes deck here that takes a bunch of failure cards. Both "Look what I found!" (2) and Dumb Luck (2) are pretty great cards in their own right. There's also stuff like Live and Learn, and Old Keyring, and you might not even mind running Rabbit's Foot if you're getting the already from Granny.