It’s allowed when it’s the observer’s own artwork, otherwise it’s copyright infringement.
And it’s not automatically casual (see the amount of RG observations in the Nature Drawing & Journaling project because illustrations are considered valid evidence of an organism:
https://forum.inaturalist.org/t/what-is-evidence/470
[Addendum: Oops, @anon83178471 already said most of this above. That’ll teach me to read all the way before typing.
Leaving my comment for the links, though. I also love plugging the Nature Drawing & Journaling project (it’s not mine!) whenever possible too.]