Why the Yak-38 outpaced its supporting technology

Would the Yak-38 Forger have worked better if it copied the Harrier completely?

Not by itself; the Soviet design was trying to implement ideas that its available supporting technology could not yet sustain.

The Forger's weakness did not come down to one bad airframe choice. Its designers attempted an advanced vertical-flight concept without the supporting technologies needed to make that concept effective. Copying the Harrier's outward form would not by itself solve the engine, control, and integration problem. Aircraft concepts succeed only when their enabling systems mature together.