Once Deranged Assistant’s ability has been activated, it can’t be reversed for any reason. If you activate it while casting a spell and discover you can’t produce enough mana to pay that spell’s costs, the spell is reversed. The spell returns to whatever zone you were casting it from. You may reverse other mana abilities you activated while casting the spell, but Deranged Assistant’s ability can’t be reversed. You’ll still have any mana the ability produced, and the milled card will still be in your graveyard.