In all honestly, if the PLA actually got their hands on a working F35.
I actually think they might get to the conclusion that it overall isn't that good and is worse than the J20 (well, might also depend on what 'version' of F35, and whether or not we take newest and latest J20).
I mean the J20 is purpose built for intercepting high value targets, while the F35 is meant to provide numbers and be fairly competent at ground attack.
The tech inside the F35 might be similar sophistication, but the J20 is optimized to take on enemy stealth fighters.
There's not as many J20 as there are F35, and J20s seem to be bad or even very bad at ground attack. The J20 perfectly suits China's doctrine of homeland defense against numerous foes with tech parity or near parity, while the F35 has been produced in impressive numbers if nothing else, which is at least a partial success of the US' stated plan for the plane.