Use parent description for product variations. By default the woocommerce product feed manager will use the description from the product variation. This code will make sure you use the parent product description for every product variation.

use-parent-description-for-product-variations.php

/**
 * Use parent description for product variations
 */
function use_parent_description_for_product_variations( $data, $feed_id, $product_id ) {

   $wc_product = wc_get_product( $product_id );

   if ( $wc_product && ( $wc_product instanceof WC_Product_Variation || $wc_product instanceof WC_Product_Variable ) ) {
      $parent_id = $wc_product->get_parent_id( 'feed' );
      $wc_parent = wc_get_product( $parent_id );
      $data['description'] = $wc_parent->get_description( 'feed' );
   }

   // IMPORTANT! Always return the $data
   return $data;
}

add_filter( 'wppfm_feed_item_value', 'use_parent_description_for_product_variations', 10, 3 );