I think the ratings don't take into account dodging. Hence HB being rated higher than BS on Snorlax for example. Regardless, an arbitrary rating shouldn't dissuade you from powering up this Blissey.
If ZH/P was upgraded a few tiers why should that make you power it up if the ingame stats are still the same?
Blissey doesn't have any terrible movesets. Some are better than others but she's still the best defender even with the least effective movesets.