Diving straight into a framework while learning the underlying system in parallel can teach you a lot about best practices and design patterns and so forth. Personally I'd say doing both is a good idea. Just make sure you learn the whys and hows for everything the framework is doing. You can vacuum up all the accumulated knowledge of the framework team.

Saying this, I have no real idea how abstracted react native is from normal IOS dev, so maybe it wouldn't be as helpful as, say, learning python and django at the same time.