How to Get Category Name When Export Products

You should change this block of code

if ( ! empty( $m_category ) ) {
   foreach ( $m_category as $id ) {
       $articles[ $key ]['Category '] = $m_category;
   }
}

to be something like this:

if ( ! empty( $m_category ) ) {
   // use $m_category->parent to check if it's a parent category or sub-category.
   $articles[ $key ]['Category '] = $m_category->name; //category name from category object
}