i just checked the rating criteria - there are a whole bunch of things, but as far as accidents are concerned, it seems to only be worth one star.
That one star is either awarded or not, based on whether an airline has had any fatalities (deaths only) in the last 10 years. If no deaths it gets the star, if yes it doesn't.
The rating criteria doesn't seem to take account of number of incidents or any other accident where everyone survived.