There are several issues in your function. Do not use backticks in the sql. Don't put quotes around field names. You use `get_results` with `LIMIT 1` when you only need the value from a single column in a single row. And you use `$meme_sku` instead of... `$post_sku`  ?
Just guessing with this example, it might work...

    function update_custom_post_meta_data() {
        global $wpdb;
        
        $args = array(
    		'posts_per_page' => -1,
    		'post_type' => 'product',
    		'suppress_filters' => true,
    		'meta_key' => '_sku'
        );
    
        $posts_array = get_posts( $args );
        $customTable = $wpdb->prefix.'custom_table';
    
        foreach($posts_array as $post) {
           
            $post_id = $post->ID;
            $post_sku = get_post_meta($post_id,'_sku',true);
                
            $result = $wpdb->get_var( "SELECT custom_column1 FROM $customTable  WHERE custom_key_id = $post_sku");
    		
    		if ( $result != NULL ) {
                  update_post_meta($post_id, 'custom_meta_key', $result);
            }
    
    	}
    }