Can Not So Fast counter Mirror Image?

Yes, but only if your opponent chooses one of your units as the target.

Not So Fast counters an enemy spell or ability that chooses a friendly unit or gear. When your opponent plays Mirror Image targeting your unit, it's an enemy spell and the chosen unit is friendly to you — both conditions are met. If they target their own unit instead, you cannot use Not So Fast, because that unit is an enemy from your perspective.

(FAQ #10501)