How should be correct recorded when the aircraft tires are "worn to limits" - as a pireps or mareps (the data is reflected in the reliability report)
A "PIREP" (pilot-report) is an in-flight report of weather or other things that other aircraft should be aware of that ATC may not know.
I'm not sure what MAREP you are referring to, the only one I can find is a "marine report" which wouldn't apply here, so I assume you mean "maintenance report".
If you are talking about a commercial aircraft, this would be noted in the aircraft log and the pilot would call maintenance to have the tire changed. This may go in the "maintenance report" you are talking about.
If you are talking about private aircraft, this would be called a "Squawk" and either the pilot would remove the tire and have it changed, or the aircraft would be brought to the closest maintenance facility for service.