You are not the Mona Lisa
with that relentless look.
Or Venus borne over the froth
of waves on a pink half shell.
In the first line, “You are not the Mona Lisa”, Billy Collins is saying that nobody is perfect. Mona Lisa can either be seen smiling or frowning, which is why Collin then goes on to say ‘relentless look.’ Collins chose to reference the painting because it is a very well know piece of art work. 

Sweet Talk