"Smooth brain" feels like the new r-word to me.
The r-word has long been used as a derogatory term for people with Down syndrome and other intellectual disabilities.
People with Down syndrome have fewer superficial sulci and gyri (bumps and folds) on the cerebral cortex.
When you use "smooth brain" to insult someone's intelligence it's really no better than the r-word.
It feels icky.