purpose of resistors in current mirror
The reason for a degeneration resistor in a current mirror is not for mismatch. The reason is only to increase output resistance.
It the texts, you'll see that the output resistance of a single transistor current mirror is Ro, and when you apply a source degeneration resistor Rs, the output resistance becomes gmRoRs.
Note that you may be confusing the issue of reducing mismatch with common source amplifiers. In a common source amplifier, you can reduce mismatch with source degeneration, since it reduces dependence on gm, however this is not the case in current mirrors.
Hope this helps