Quality ranking is one of Meta's three ad relevance diagnostics (alongside engagement rate ranking and conversion rate ranking). It compares your ad's perceived quality against other ads that competed for the same audience and reports a relative grade — above average, average, or below average. "Perceived quality" is built from real user signals: feedback on the ad, hides and reports, and Meta's assessments of low-quality attributes like clickbait, engagement bait, and withheld-information tactics.
The relevance diagnostics matter because they describe how the auction sees you. Meta has been explicit that higher-quality, more relevant ads tend to cost less to deliver and reach more of their audience. A below-average quality ranking is the delivery system telling you that people who saw your ad responded worse to it than they did to your competitors' ads — and that you are likely paying for that gap on every impression, which eventually surfaces in cost per result.
What does the comment section have to do with a quality grade? The signals quality ranking is built from — hides, reports, negative reactions — are responses to the full ad experience, and the thread is part of that experience. An ad whose comments are a scam bazaar invites the hides and reports that negative feedback measures; an ad whose prompt is pure engagement bait is flagged as a low-quality attribute in its own right. Cleaning up the thread does not directly edit your grade, but it removes standing provocations for the user behavior the grade is computed from.
The honest framing: quality ranking is mostly won with better ad creative and better targeting. Comment moderation is the supporting act — it stops a good ad from being dragged below average by a conversation the advertiser never looked at. If your creative tests well but the diagnostic disagrees, the thread is one of the first places worth checking.