@[email protected] I've added my own caption to uncaptioned pictures before, and left out account name as I felt like it wasn't actually a reply to the OP, but rather a personal addition to the thread that would make me feel comfortable to actually boost the post.
I see my caption reply as a courtesy to my followers who see the boosted toot, not as a courtesy to the poster, especially since some see it as a passive-aggressive attack.
@[email protected]
Also, due to the nature of federation, I can't be sure another user hasn't already provided a caption too. As such, the OP could end up getting tagged several times by different users with basically the same content since they can't see each other's replies since they aren't federated with each other. (Or because they were too lazy/forgetful to check the thread for earlier caption replies.)

@[email protected]
Finally, if an OP actually cares about having captions, they can subscribe to one of the various caption reminder bots¹ to be reminded when they miss one.
Alternatively, if they don't have the spoons/time to describe, maybe they can hashtag it with something like #CaptionMe, or an appropriate emoji.

If they don't care, then they shouldn't care either if I reply with one without mentioning them. :)

¹: https://botsin.space/@PleaseCaption, https://mastodon.social/@alt_text, and I bet there are more

Please Caption Bot (@[email protected])

6 Posts, 8.59K Following, 12K Followers · A bot that reminds you to caption your toots' images and videos

botsin.space