Is that plate an insult? Or is it his way of giving compliment without saying it?
My best guess? A compliment.
Japanese aesthetics had an emphasis not on grandeur and razzle dazzle. They often seen simplity as beautify, and art works that were simple, appearing as if primitive, is seen as beautiful.
It's beautiful not in the sense that the pottery is symmetrical and evenly shaped. On the contrary, the work is praised because the artists tried to make it look as close to nature as it can be. Think of ... a rock you see on the road for example. You can look up google for pottery and see the examples I'm talking about.
So Kaibara offered praise with a genuine work of his.