There are clear benefits from fiber consumption for bowel function and blood sugar control in people consuming a diet high in carbohydrates. But when in nutritional ketosis, it appears that many of those same benefits are provided by the ketones you make in your liver, thus making dietary fiber less important when you are on a well-formulated ketogenic diet.
There are times when fiber supplements may be useful, but it is a case-by-case basis and only considered after ensuring that adequate sodium and fluids are consumed. It should be noted that vegetables serve several important functions in addition to fiber; they are are a source of vitamins and minerals such as potassium and magnesium, as well as a vehicle for getting adequate sodium and fat. Consider adding some nuts and crunchy vegetables as snacks during the day.