add_filter('frm_field_classes', 'add_input_class', 10, 2);
function add_input_class($classes, $field){
  if($field['id'] == 25){ //change 25 to the ID of your field
     $classes .= ' my_class';
  }
  return $classes;
}

Disable the date popup on mobile devices, but keep it everywhere else.

add_filter( 'frm_field_classes', 'frm_remove_date_field_class', 30, 2 );
function frm_remove_date_field_class( $class, $field ) {
  if ( $field['type'] == 'date' && wp_is_mobile() ) {
    $class = str_replace(' frm_date', '', $class ); 
  }
  return $class;
}