Ratings on DoorDash are no different from ratings on top sites like Yelp, Google, and Facebook. Guests decide which restaurants to choose based on ratings. The restaurants with the best rating will be chosen more often. The best rating is not always 5 stars or even 4 stars.
The best thing is relative to you and your competition. I also checked my email thinking they would send a review link after delivery; none of my previous orders had follow-up review emails. Because the company doesn't review or endorse customer feedback, the comments are private, meaning only restaurants can access them. They receive ratings from restaurants through the app; for reasons beyond me (and ratings from harmful drivers), computer and email ratings give you only one option: evaluate your driver.