Although md5 mismatch is added by a bot, it doesn't mean you shouldn't add it yourself if there are actual differences in the file you're looking at. You don't need to make any comment in addition to adding the tag either. The bot is only there to make it easier to catch these but it isn't fast nor does it catch all instances, even on Pixiv. I've had to add it myself in several cases.
If md5 mismatch applies to the post you're looking at and it's not already there, please add it. It also helps with some Danbooru-helper userscripts such as the Pixiv one which highlights bad revisions in yellow to stop people from uploading worse-quality images or in purple if there's a revision available that hasn't been uploaded or checked yet.