The official rating of a race just indicates which horses it is intended to be for e.g. 0-90 would be for horses whose official rating falls in that range.
How good the race actually ends up being will depend on which horses are actually entered, and how well they perform on the day. Determining how good a race was after it has happened is a very subjective matter, and different people will place different weights on a huge number of different criteria such as official ratings, times, jockeys, the draw, the going etc.