Whether grappled or not, the enemy can always shut their eyes and attack blindly into your space. If they do, they don't have to contend with mirror image, but they do suffer disadvantage on the attack due to blindness, and they grant advantage to anyone attacking them.
If they're prone, they have disadvantage on the attack and grant advantage to melee attackers anyway. The only drawback at that point is that they would grant advantage to ranged attackers as well (canceling out disadvantage from being prone).
If they're prone, they have disadvantage on the attack and grant advantage to melee attackers anyway. The only drawback at that point is that they would grant advantage to ranged attackers as well (canceling out disadvantage from being prone).