If you're subscribed to Pro or Patron, you can tag multiple comics at once. From your Collection page, you'll see an orange button with a plus sign at the top of the page, click this to go to the Bulk Add/Edit feature. Here, you can quickly search and multi-select the comics you'd like to tag. On the Tags tab, enter the phrase/tag you'd like to put with these comics and then hit 'Add' next to the text field. Once you're done, click the Add/Edit button on the bottom right of the modal to save your changes.