Yes, Jyn's turn meter removal very much so affects the Rancor. After Commander Luke she's probably the single most effective character at keeping the Rancor in check, so getting her is definitely a good way to improve your performance in that raid.
As for potency, as others have said, she doesn't really need any - especially if you're running her alongside someone who gives Tenacity Down. The tenacity penalty that gives is so severe that potency doesn't matter, the resist chance will always be the minimum (15%). And that goes for everyone, not just Jyn.
Even without that though, after a few crits she'll have built up enough potency to be good. You'll want someone else who also removes turn meter in case she does get resisted - especially at the start of the raid while her potency is still low/before the Tenacity Down gets applied - but there's plenty to choose from. I've been soloing with her, Han Solo, Thrawn, and Wedge + Biggs lately myself.