so my force consists of Monster A, and GG moleman. Monster A attacks and hits an opposing character. At this point because of this rule "DAMAGE DEPLETION MODIFIER
Each time an attack resolves, modify the attacker’s
damage value by -1 (minimum 1) until the end of the
turn." Monster A's damage is at -1 already. Moleman 1 mindcontrols monster A to attack and hit the same opposing character. So, does he now have -2 to damage because of the depletion modifier?

Short answer: Yes
Long answer: Yessssss

But AFTER actions resolve. So if he somehow makes an attack, it will be -2.

Monster A's first attack is at normal damage.
Mole Man MC's Monster A to attack again, this attack is at -1 damage.
If he would attack again he would be at -2 damage and so on.

That would reset at the beginning of each players turn, so if your opponent hit Monster A with Mind Control his attack would start back at printed value.

So this means if I MC an opposing figure twice in the same turn the DDM would kick in?

Yes, it would.

That's going to take time to get used to, for me...

Although if you want to use a mean trick, you can include an Outsider on your team and use the TA on whoever is going to be MC'd... no DDM applies as their stats can't be modified.

True, but it's especially delicious when your opponent used Perplex to up their attack to get you and then on the ensuing turn you get the benefit of their still perplexed up figure via Mind Control.