For example, in this recording, Iris sings and appears in video too. Can I add info about that with this relationship, or it is undesirable?
As far as I can see in the description of this rel, no:
This indicates that an artist appears on a music video, but doesn’t actually perform on the audio track.
If it’s true, then I want to ask why?
I think it’d be better if the answer were yes, since sometimes artists don’t appear in their own videos (common for electronic music and unofficial music videos), but I believe the current general guidance is no
KPop OST videos this is common for people wanting examples. Those music videos are usually clips from the show/movie and the artist singing the song rarely appears(for ones I’ve watched at least)
In my experience, it’s generally fine to use the “appears on video” relationship for people who perform or sing in a video, as long as their contribution is significant. For example, if they’re clearly featured and have a notable presence, it makes sense to link them. However, if they’re just briefly seen or in the background, it might be better to leave it off to avoid clutter.
The point of this relationship is to indicate cameos and the like that would be interesting info, with the assumption that for most music videos, the performers are assumed to be part of the video.
Whether that assumption is correct or not is a different thing, and it’s possible it might make sense to revisit the relationship - although I still feel it would feel a bit redundant for most artists who do indeed appear in all or most of their own music videos.