This is possible for developers to implement via the JSON API or Conversation.js. Is it done “somewhat easily”? Well, I guess it depends on who you are. 
It should be trivial for a developer, but it’s probably pretty challenging to anyone lacking programming skills.
And, there’s the question if this is a good idea… If your index page lists, for example, 25 blog post, you have to do 25 network lookups to figure out how many replies each post has. So it’s a bit heavy on the network, and something @manton might object to. 
Also, if you fetch this information during your blog’s build step, the reply count will go stale between builds. This could be worked around by fetching the reply count on the client side using JavaScript, but that’s even more inefficient from a network perspective.