You can say:
– Beauty is only skin deep.
– Appearances are often misleading.
– Looks are decieving.
– What really matters is invisible to the eyes.
– Anything essential is invisible to the eyes.
– One sees clearly only with the heart.
– Looks aren’t everything.
– All that is gold does not glitter.
– Don’t be shallow.
– Don’t just look at the surface.
– Don’t just scratch the surface.