This is a great question. The theory of forms and the idea of imitation (mimesis) is very different, but they are related in a sense. Let me explain.
Plato's theory of the form can be culled from theRepublicand thePhaedo. However, it should be said that he does not give a systematic treatise. Some characteristics of the forms come from thePhaedo are as follows. The forms are unchangeable (78c10), eternal (79d2), divine (80a3) So, we can say that there is a ultimate "good," ultimate "justice," and the like. Earthly justice and earthly good are just approximations of these true forms.
In light of this, Plato argues that philosophy is necessary to cut through the images to get to the meditation of the true forms. In other words, through dialectics, a true philosopher will be able to get to perceive what is truly good, beautiful, and just.
The idea of imitation is usually seen in connection with poetry. Plato, as you know, banishes poets in book 10. The reason for this is twofold. First, poets are not philosophers. So, they do not know about the true forms. Second, their work makes people imitate what is not true. This makes the citizens worse, because they are now farther way from the forms.