Bill wrote:
Trump is carrying out an ad hominen attack based on sarcasm. (Or one of his advisors in this case - even this level of 'wit' is probably beyond the man himself). Thunberg is making an unhumorous, unironic, unwelcome, forthright and essentially accurate statement.
Still, it's good of Trump to actually propagate her message. And of course an ad hominen attack in response to a logically reasoned argument (whether you agree with it or not) is a pretty sure sign that the attacker has no counter-arguments that will hold water.