Hi,
It has been found that at Re <20,000 (like many indoor R/C models and foamies), a thin flat plate actually generated more max lift than a Clark Y airfoil.
However, a properly designed airfoil optimised for that operating condition would probably outperform both of those.

One reason is that the air molecules don't "see" a thin plate airfoil the same way we see it.
Some air approaching the wing goes over the top and some goes underneath.
The point where the air splits near the wing's LE is called the stagnation point.
As the air approaches the LE of a wing with a positive AOA, it starts sensing the high pressure under and the low pressure above and some of the air just below the LE manages to sneak over the top.
This causes the shifting of the stagnation point to a spot somewhat below what we consider the wing's LE. As the air molecules hooks its way back around the LE and over the top, it rounds the shape as it goes, so the upper surface flow gets a long, curved path from the stagnation point to the wing's TE.
The result is that the air molecules "see" a thin plate airfoil with a positive AOA as it was a cambered airfoil.