That's like saying Micah was our best CB because he was our quickest so could bail himself out when people go past him...Much rather someone who can prevent them going past in the first place at which Mendy is no better than Kolarov.
Micah was a right back.
How many games of Mendy's have you actually seen? From the ~10 Monaco games I saw last season he's a much better defender 1v1, has the pace to catch up when caught up the pitch and his positioning is something Guardiola will work on. Being caught high up the pitch is a completely normal thing for a fullback playing in a system as attacking as Monaco's last year, and having the pace to recover when that happens doesn't make you a bad defender.
The idea he's worse than Kolarov defensively is ridiculous, I can only assume this is some weird thing because Kolarov is leaving.