The power and toughness of the Ooze token is determined when Miming Slime resolves. It won’t change as the greatest power among creatures you control changes.

The value of X is the greatest power among creatures you control when Miming Slime resolves. If you control no creatures at that time, X will be 0, creating a 0/0 Ooze token that will be put into your graveyard as a state-based action (unless something else is raising its toughness).