The Google Play Store is pretty secure, but fake reviews are still a problem (like they are across many services). Google is working hard to improve the review process and has announced that it has improved fake review detection.
Google says that if developers are using an agency to promote their app, that they should make sure the agency adheres to the Google Play Developer Policy, otherwise the reviews might be marked as fraudulent.
Hopefully this renewed effort will cut down on fake reviews that make poor apps look good. There are enough bad apps on the Play Store.