It is a misunderstanding that most Americans oppose the ACA.
Most Americans understand the need for universal healthcare coverage. Those who don't are in the minority, but are vocal and people in other countries seem to believe they are representative of the majority.
Obamacare is a nickname created by opponents--the same opponents who severely limited and sabotaged its success. This was done deliberately to pave the way for them to come in and pass a similar but differently named law that does the same thing.
Extremists say they want to totally repeal it because it didn't work. Intelligent people understand it takes longer than a few years to make such enormous reform "work", and the flaws in the ACA (engineered by opponents) needed to be fixed so it could succeed.
One example of why it takes a long time to determine the ACA's success is the fact that the uninsured had been in poor health for their whole lives. Suddenly they had insurance and got care. Costs, as expected, would go up during this time.