The status effect of Mining Fatigue only affects the actual rate of mining a block, and does not relate to the actual swing of the player's arm.

It would be logical that the player would not be able to activate another "punch" if the animation is half way through.

To replicate:
1. Apply mining fatigue of any level, higher is more noticable.
2. Spawn any mob.
3. Using any item, or empty hand, punch at the mob.

It can be noticed that the mob can be hurt as fast as the damage buff will allow it to be.