I believe you can upgrade the Hammond Creation Lab to have higher viability, up to 30%.
Meaning, if you have a 100% dinosaur and you heavily change it's gnomes and it drops to 75% it will still be 100%.
I believe it is the chance that the dinosaur will develop/incubate correctly.
So something with 30% viability might fail to incubate properly whereas at 90% it has a good chance of making it through incubation.