No. God is neither the form nor the matter of other things because He is the first cause whose actuality is primary and essential (i.e., He is the being of other things, but not their essence).
Note that Aquinas here, in a rare display of disgust, calls the theory of David of Dinant that God is prime matter the "most stupidly" formulated theory of all. (In the Latin: "qui stultissime posuit Deum esse materiam primam.")
The "most stupid" theory has it completely backwards: as if God were pure potentiality (viz., prime matter) rather than Pure Actuality!