1. onX lets anyone with a least a free account add a trail report to an existing trail that has been already been mapped and published to the system.
You can add a trail report to that specific trail that states why you believe the trail is overrated or underrated. You can also add in any specific remarks that describe changes to the trail that you observed, along with adding your own photos.. etc..
If the trail description / rating is woefully bad, you can contact onX and advise them of such.
Just recently, after getting onX admin approval, I went out and remapped an entire National Forest (15+ trails) because the previous mapper did a horrendous job. That persons reports read like a Yelp review, the trail descriptions were way off and terrible, and the photos listed were all personal type photos that showed no trail conditions whatsoever.
How did those terrible trails slip through review? They apparently were done at the very beginning of onX 'sTrail Guide program where they were still trying to dial in the process..
2. onX uses their own trail rating system. You can see detail of their rating system, with photos in my post here: (post #7)
https://thebronconation.com/trails-trips-n.64/bronco-trail-app-t.21435/#post-329926
Read through that entire thread if you have the time as there is some good info in it re: the various mapping apps.
Basically, onX Trail Guides go out and track / map trails. It is then uploaded to the onX system by the Trail Guide with trail description, photos and a trail rating. Once uploaded, it must be reviewed and approved by the onX mapping team. If the trail is approved, the trail is published. They have guidelines that Trail Guides must follow like trail length, photo requirements and trail description requirements. etc.