I think "was brutally honest with them" describes it best. I don't think President Trump went to speak at the Libertarian Convention to disrespect them, on the contrary, speaking at their convention was a show of respect.
And the idea that he was "mocking" them is absurd. He went there to win them over and made a very good attempt at it.
He went there to win their votes, even ask for their nomination. Might not get the latter, but I bet he won some votes.
Yes, I agree. “Disrespecting them” isn’t really accurate either. I’ll go with “brutally honest”.