I think the relevant feature would be the ability to add custom calculated fields. You would then be able to calculate anything you like from existing data in the same record. I suppose that hasn't been done because there's no compelling need. Considering the data is coming from websites, it's usually in a form that requires no calculation.
Calculating an average rating would be one of very few things that could be calculated. Even then, an average rating is of dubious value because the individual ratings are fundamentally different. Displaying it instead of the individual ratings would lose most of the information and very likely be misleading (e.g., where some of the component ratings are missing). It may be no more than a meaningless distraction if an important part of the information is in the differences between the ratings (e.g., user vs. reviewer ratings).
