Maybe Zomato should make me rate for more granular aspects of the restaurant, e.g. Food, Service & Ambience, and then based on those scores automatically calculate an overall rating.

Yes, for the user, this is not as easy as just giving one rating. To solve this, they can just make the rating for food mandatory, as that’s the most important aspect. The user can optionally choose to rate for more aspects, which will affect the overall score. This will not only make the system better, but is also helpful for the user. One need not go through every review and photograph to find out more information about the restaurant.

What say, Zomato?