An even smarter way (in my opinion) of getting 5 star reviews is by showing the dialog only after x hours of use. Users most likely to rate badly will have the app uninstalled before the dialog shows.
I can see how you'd see this as providing a positive bias, but I see it more like getting a chance to see if you can't help the customer out before they give up on your app. It also reminds the customer that there are people on the other end - so even if the issue can't be resolved, and you still get a one-star rating, the level of vitriol seems likely to be reduced - something all too easy to forget when angry-reviewing.
Of course if they just take the feedback and dump it, that's a different story - but again, I would think anyone with that experience would still leave the negative review.
TL;DR - too many downloaders use negative reviews as a combination support request and cudgel. I think this is a reasonable defense against that.
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...
Otherwise I don't mind such dialog boxes.