As an important element, its no surprise that salt became associated with admirable qualities like intelligence or being interesting.
So while a dish might essere dolce di sale (lit.) to be sweet of salt (fig.) to need salt, be bland, you might also say it of somebody who is particulary dumb or gullible.
If the person is a little "dense" or has a hard time understanding things, you could also say that they were "behind on salt" or indietro di sale with a similar meaning.
If you fall asleep during a film, your mind wanders during a conversation, or can't get through the chapters of your book, it might essere senza sale (lit.) to be without salt or fig.) to be dull, insipid.