I see limited usefulness for this feature since unlike mentions of posts/users/groups there wouldn't be any kind of notification associated with the feature.
To increase its benefits I'd say it should come with a hover card that gives statistics about the tag, and maybe includes a button to immediately start a discussion in the tag?
From a purely aesthetical standpoint, this seems more akin to rich link previews, in the same way that it would be nice if links to discussions or profiles were automatically turned into previews as well. This is kind of what my Rich Embeds extension does, although the results are cached so it doesn't show up to date information, and tag links are pretty boring since they would just have the tag name and the icon of the website, but no sign of the tag color. That's something I could add through the Flarum integration already available for discussions and profiles.