Amazingly, after all this time, the F-35 isn't better than a B-2 bomber or the F-22 in general terms of stealthiness. I suppose all of this is rather subjective matter dependent on various technical factors.
The laws of physics constrain what is possible in terms of RCS reduction. The stealth technical/engineering improvements are in the "mechanics" of RCS reduction. For example, the F-117 required extremely maintenance intensive "RCS Putty" to seal the various panel seams. And applying the putty properly was as much an art as a science. It was a very maintenance intensive aircraft. The B-2 requires air conditioned hangars to maintain its RCS properties and there has been public discussion of RCS degradation after flying through rain. F-22 has its share of maintenance quirks to achieve full RCS reduction. The F-35 is (allegedly) the first stealth platform whose RCS features are not maintenance intensive and whose characteristics are compatible with a typical carrier environment.