The CSV report in Manage Returns can be filtered by SKU and Product Name, but that data cannot currently be abstracted to a higher level category or subcategory (i.e. Shirts or Button Downs). Exporting a CSV for a given timeframe will include all statuses of returned items (authorized, in-transit, delivered, completed), then filtering by desired SKUs or Product Names, and summing total number of results will give total returns for a given SKU or group of SKUs in that timeframe.