You can sign IMPORTANT and shake your head for negation.
Or, you can use the sign for UNIMPORTANT. The sign starts like IMPORTANT but when your two F-hands meet at the top you immediately open them into 5-hands, palms facing down, and spread them apart with a dropping motion. Use the appropriate dismissive facial expression, of course.
Depending on the context, you can also use NOTHING-TO-IT, or WHATEVER, or various other ways to signify the unimportance of an object or activity.