Mutating the enchanted creature won’t cause it to gain abilities. This is because mutate modifies the creature’s base characteristics, while Mystic Subdual applies to the resulting creature.

If a creature has power and toughness written as */* with an ability that defines its power and toughness, its base power and toughness become 0/0 when it loses all abilities. Mystic Subdual then makes it a -2/0. If its power and toughness are written as */*+1, it’s -2/1, and so on.

If the affected creature gains an ability after Mystic Subdual becomes attached to it, it will keep that ability.