A higher Vgs (i.e. a higher current) would give better mismatch for a tail current.
Intuitively, this can be explained by the fact that for a given delta Vgs mismatch, higher Vgs would mean smaller impact ( since selta vgs is small compared to vgs, the corresponding delta I would be likewise small).
A derivation of the equations can be found in Grey & Meyer's book "Analysis and Design of Analog Integrated Circuits, 4th Edition"