Did the Council of Nicaea decide that Jesus was God?

No. It addressed a specific dispute about how to describe something Christians already widely believed. It didn't invent the idea in the first place.

In 325, a churchman named Arius was teaching that Jesus, though exalted, was a created being—not eternally equal with God the Father. That view had real support among some bishops. The council was called specifically to settle this disagreement. The large majority of bishops present affirmed that Jesus shares fully in God's own nature, producing what's now called the Nicene Creed.

Worship of Jesus as divine already existed long before this. A Roman official writing around 112—two centuries earlier—describes Christians singing hymns to Christ "as to a god." This wasn't a new idea Nicaea introduced.

What Nicaea actually did was settle precise, careful theological language for an already-widespread belief, in response to a real challenge to it. It didn't decide the question from a blank slate.