Of course, it is. Divorce is always a sin in the sense that it represents a failure to attain God's ideal for marriage. That's the very definition of sin - failure to achieve God's ideal. Something went wrong. Somehow the marriage failed.
Divorce always represents the failure of a marriage, and as such it is sin.
However, the sin is not necessarily in the act of getting the divorce but in the actions that caused the marriage to fail. For Jesus, there are no excuses acceptable for failure. A failed marriage is always a sin. He sets the bar all the way up. Now we must remember that the setting was a theological contest, not a situation of pastoral care for real people.