crazycells that's a good question that probably bears further research.
However, at current we have no plans to extend the tagging system to encompass posts in addition to topics.
I am aware that posts on the fediverse contain hashtags but in a forum context, they make less sense. That is why tags have traditionally been at the topic level.
Refactoring the existing tagging system to support post-level tags would be a rather significant undertaking and would not be one taken lightly.