Presumably the content of the message is irrelevant to Tor. If the humanness of the content matters then the game is over anyway because that means it’s being read by a third party.

What matters for Tor is plausible looking traffic and uninteresting people doing uninteresting things with it. Then you can’t use traffic analysis to identify Tor users as points of interest.

If everyone on Tor is evading an oppressive government then oppressive governments can just use Tor to identify the people evading them.