How have Donald Trump and other US politicians responded to the victories by the populist Five Star Movement and center-right coalition parties?
Google suggests that most haven't noticed yet. I did find an article talking about Steve Bannon. He is quoted as saying
This is just a populist victory and should send a massive signal to the permanent political class in Rome and more importantly to the permanent political class in Brussels that people want change
You can read more on Die Weltwoche.
I would suspect that Italian politics is low on the list of priorities of most in the United States. We might see more in a week or two, once the media has a chance to learn more and ask questions of US politicians.