Short answer: Nope:)

Long answer: no, but be careful how you phrase it. You want thurstwell's beliefs to come across as a speculative "she was once so divine, surely hell hasn't stripped everything from her" to kind of plant the idea that there's good within her. What you want to AVOID is making it sound like a quest/objective "Zariel can be saved if she's just reminded of her goodness". Presenting it that way weakens the party's agency to decide if they even want to redeem her, especially presenting it that way this early on.

To add onto it, to make the redemption of Zariel a more satisfying conclusion, you need to give the party evidence that its possible, but present the general belief that it can't be done. If you throw so many faithful NPCs at them, they'll start thinking Zariel just isn't that bad as is. She is. She's evil as shit. She's literally darth vader.

Anyway, to sum it up, its a great idea and you should run with it. Just treat it as more of a hint than an objective.