We are trying to find a way to hide 'Is Helpful' question on KB articles for a particular category. Please let us know if there is a way to implement this.
There in no out of the box method that can turn off the "Is Helpful" button per category, only a way to turn off the button for every Knowledge Base via the "Knowledge article view properties". However, what we can do is submit an enhancement request on your behalf if you do strongly desire the removal of the button. You can also limit the user roles that can see the "Is Helpful" button, but only for all of your knowledge bases, via the property settings.