It does display, but for a user in a test environment with 43108 messages and a 10968 reaction score the ratio that it is spitting back to me is 4310.8 – I note this is just the post count with the decimal place moved one spot. I believe the value should be more like 0.2544. We're running v2.2.1 in the test environment.
Edit: I think it needs to be "$user.message_count|number" and perhaps the other way around?
Been thinking about this and it only starts to fulfil its real potential of usefulness as a meaningful metric after so many posts.
For example someone could make just one post of a very nice video, they could easily get half a dozen likes for that one post so have a huge Like/post ration that isn't really representative of how useful a member they are.
It only really makes sense once a user has a decent amount of posts, and only then if the reactions are in relation to what they have to say, rather than if they just post videos and memes.
So maybe I'm cooling off the usefulness of this for now.