I think that technically it is a bit more involved than just ping-pong messages. There are all sorts of messages that the server(s) send and that mIRC uses under the covers to maintain the state (e.g. the channel users list) but which are not users actually speaking, and some of these result in channel window lines being added e.g. for joins/leaves/quits/kicks, whilst some are purely internal. (Try "/debug @debug" to see the raw traffic.)
But I certainly think that the tray icon animation could be more specific about traffic, and it could also be used to indicate whether there has been traffic that resulted in green or red highlighted messages since the last time the window was visible and active. And maybe have a scriptable extra alert (e.g. flashing the entire icon).
