Aircraft most certainly do fly around above Mcrit although some may be restricted to speeds at or below Mcrit when is Mach Trim is inop.
Why would a Mach trim system operate at speeds below Mcrit? It is fitted to pre-trim to compensate for the aft movement of CP due to the formation of the shockwave.
It's a pretty safe bet that Mcrit is at or close to the Mach number at which the Mach Trim becomes active.
Of course some aircraft happily fly around above Mcrit without a Mach trim system.