Is it allowed to offer incentives for 5 star ratings on the Google Play app store? I started using an Android app that is offering in-app incentives (unlocks) in return for 5 star ratings on the Google Play app store. Is this against Google Play's terms of service?
It is a violation of Google Policy

Highlighting one part of policy violation, related to what you say

Here are some examples of common violations:
Asking users to rate your app while offering an incentive

Edit:

For other violations see Google Play Content Policies

For reporting use Report Inappropriate Apps