“Bad faith” is a common, normal term that has a well-accepted concrete legal meaning [0].
In this context it basically means posting a rating or review which doesn’t match your actual opinion. For example, crowd-sourced vote brigading (where you don’t have an opinion at all but are just voting in the way someone else told you to vote). Similarly, reviews along the lines of “This app only deserves four stars, but currently it has an average five star rating so I’m going to give it a one star review to try to bump the average rating down toward what I think the average should actually be” would be reviewing in bad faith.