Santa Barbara County officials estimate that only 15 percent of the residents ordered to evacuate heeded the warning, ahead of a dangerous storm that has killed six people.   Several homes were swept away early morning on Tuesday when mud and debris roared into residential neighborhoods in Montecito.   They originated from hillsides stripped of vegetation during a recent wildfire.   About 21,000 people were under mandatory evacuation orders by mid-Monday.   County spokeswoman Amber Anderson said only 200 of the 1,200 residents contacted in person had left by the evening.