Users with negative experience are more inclined to leave a rating than users with a positive experience, which makes the ratings not reflecting the reality of the overall user experience in the first place. (http://cdn2.hubspot.net/hubfs/232559/The_Mobile_Marketers_Gu...)

This technique is a genuine way to encourage sharing positive experiences about the app. In the same time, it offers a chance to the app provider to improve a bad user experience.

Whether it's a dark pattern or not, I think really depends on the motives, are you genuinely trying to make the app better or are you only interested in the people's perception of it.

Here is a great article on the topic which discusses the same issue: https://www.smashingmagazine.com/2014/06/a-better-way-to-req...