https://www.pro-football-reference.com/years/2019/defense_advanced.htm
Based on passer rating against in 2020, here's where NE's CBs rank (this list includes corners, safeties, and linebackers, all whom have been in pass coverage - btw, there are 268 players listed):
JC Jackson - #24 - 29 targets, 18 comp, 252 yds, 62.1%, 1 td, 4 int, 61.9 rating
J Jones - #44 - 39 targets, 21 comp, 53.8%, 264 yds, 1 td, 1 int, 73.0 rating
Gilmore - #46 - 30 targets, 16 comp, 53.3%, 218 yds, 1 td, 1 int, 74.0 rating
J McCourty - #250 - 17 targets, 11 comp, 64.7%, 179 yds, 3 td, 0 int, 139.5 rating
So honestly, the first three guys have been just fine. Very good, in fact. JMac, on the other hand, has been a *disaster* at corner this year. An absolute disaster.
It feels like Gilmore has been bad this year, but that's only because he was otherworldly last year (and btw, in both 2019 and 2020, Jackson has been better).