Was upholding the mandate in the lower courts judicial activisim?
The Prez and dem leaders were in favor of those lower court decisions but now is the supreme court on their pending decision judicial activism?

No.

Find me where in the Constitution, supported by the Federalist Papers, it says that the Federal Government can mandate that you buy a product.

If we wanted Constitutional government, we'd realize that Medicare, Medicaid, Social Security and most of the Departments of the Federal Government are unconstitutional.