Trump used the plural form.
He wasn't just referring to Omar.
He was saying that AOC, Tlaib, and Pressley are also not really "from here".

Sure, he might be saying what everyone else is thinking.
But that doesn't make it good!
If you've never had a schoolyard bully tell you to "go back where you came from!" when you complain about his bullying, then, consider yourself lucky.
It definitely gets said. The Trumps definitely said it to others when they were in grade school.
The victims had to learn that the perps were ignorant.
The perps never got their forced learning opportunity, and some of them never matured.
One of them ended up as President.
None of that justifies anything. Noah was said to be the last righteous man. Those who didn't get in the boat probably thought, "oh, I'm just doing what everyone else is doing and what everyone else is thinking"

---