I think that number is a bit off. Yes, there is overhead when charging a car to run its battery management system, heat losses in the wiring, etc. But it's not 20-30% of the ~kilowatt of power you'd run through level 1. A quick search says that 20% loss is at the higher end for level 1 (probably 15% on the lower end) but even level 2 has about a 10% loss.

The bigger issue is that level 1 just doesn't have nearly as much power as level 2. Most cars charge at level 1 at 8-16 amps. Most level 2 setups charge at a few times that, plus the voltage is doubled so the total power ends up being about 10x as much. But that's not to say everyone needs that power either. Honestly, for the average driver it's quite easy to make level 1 work.