Is there a standardized rating system for game difficulty?
There is not.
Typically, difficulty scaling has to do with two major factors:
- Your survivability
- Enemy survivability
For example, Skyrim scales its difficulty by changing the amount of damage creatures do:
In terms of actual standardization, no, there really isn't. It's up to the developer to determine what they consider as "difficult" and what they do not. Play data can be used to determine future adjustments.