Physically, there is a relationship. Provided everything else is constant, of course.
But if you're talking about the MEASURED 'g' and the MEASURED descent rate, then things get a bit more complex.
Generally, you'll be looking at the FDR/QAR data to determine if its a 'hard landing'. Both 'g' and descent rate (or rad alt, which is probably what's being used to determine the latter) are only recorded at discrete intervals - almost certainly not simultaneously. And the impact is of course a very short duration event.
So its very easy to miss the peak 'g', or to not get any data right at the instant of impact. In such circumstances, there's no guarantee of a correlation between the measured parameters.