You can leave comments for any card on any page - the little cartoon bubble/caption icon is displayed under the card image.  It's between the magnifying glass and the heart.  
 
See it?
 
Leaving a comment for an Artist is done from the storefront only via "Testimonials >> Submit Yours".

I know what you mean Nicole. . . it's the same when you click on a widget, you get to ONE card, ready to buy. . . and there is no place to leave a comment.  There may be the same card on the left in the "4 up" box with possibly others by the artist, but more likely others in the category. . . the artists' names are under but you can't click on it to see more work by that artist. . . there's no link.  If you're cleverly tenacious, you can copy the "by" name into the search box and maybe get to that artists store, but not always.  If we as artists, who have been here a while (and know something about how it works) are frustrated with the navigation, I'm sure visitors must have some of the same experience.
 
-E(g)

I'm not sure I'd notice a card comment if someone left one, and I'm not sure we can "manage" them, such as deleting them if they are nasty.

Ah ... thanks.  I did have one "review" which turned out to be a comment only.  I had to scroll through all my cards to find it.  I was able to read it but not edit it or delete it.  
 
 
PART II
=====
I do see our Store Report also shows the reviews, so they are easy to find but I don't see a way to edit or delete them.