In case of the US, I don't think it isn't that China isn't good in propaganda field, but China see not much benefit in doing so. China isn't interested in poking the enemy without any tangible benefit unlike what the US is doing today to China. Besides, the US under trump has committed blunder after blunder, like sanctioning the ICC judge yesterday. Sometimes, interfering when your opponents are committing blunders could get you the unintended result. Better leave them dig their own holes deeper without them realizing.