Imma be real with you man dropping an entire realm on American soil isn't going to be fun, diplomacy won't be easy because Star needs convince the world that mewni has it's own government and must be treated as a country because heaven's know what the United States would do to the everyone if Star only turn mewni as an American state.

Xenophobia would be rampant because you just introduced a variety races to a large majority of people who only see this kind of things on fictional media people would be afraid of them a would take up arms believing this is some kind of alien invasion. conspiracy theories would float around the web. And don't forget that this poor people would be a major target for human trafficking.