When cooking mince in the slow cooker it's best to thaw the frozen mince out first in the refrigerator so that it cooks more evenly in the slow cooker.
If you place the frozen mince into the slow cooker without thawing it first it may not reach a high enough temperature soon enough which can allow bacteria to grow onto the frozen mince.
But if you do put frozen mince or any other frozen meat into the slow cooker the mince or any other frozen food may need to cook longer in the slow cooker.
I would recommend thawing the frozen mince out first at least some so that it cooks much better and more evenly and the risk of getting bacteria onto the meat will be lower.
I always just allow any meats including the mince to thaw in the refrigerator overnight the night before I want to cook it.
Then it's thawed enough to just place it into the slow cooker.