It depends. I do not think mental cruelty is grounds for divorce if mental cruelty concerns the way a mate twists the toothpaste tube or hangs stockings in the bathroom. That type of mental cruelty has been defined in so many different contexts it has no meaning.
However, I do think physical brutality and abuse, and mental abuse of a nature that endangers the person’s mind or body, are clearly grounds for divorce. The Pauline privilege, which I mentioned earlier, (1 Corinthians 7:15) permits divorce on the grounds of desertion by an unbelieving spouse. For mental cruelty to be grounds for divorce, it must involve conduct which makes it impossible to live with the spouse without endangering oneself.
The sort of cruelty I have in mind would not spring from a criticism of a soufflé or a brother-in-law. Minor irritations need loving attention, but should not be allowed to rupture a holy relationship.
Obviously, a couple composed of two born-again Christians does not fall under the Pauline privilege. Divorce and remarriage for any reason are truly unthinkable for two people who sincerely love God and are trying to serve Him.