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.
